Acquisition - Business Acquisition, Unaudited Proforma Result of Operation Information (Detail) - Central Bancorp, Inc [Member] - USD ($) $ / shares in Units, $ in Thousands  | 
3 Months Ended | 9 Months Ended | ||
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Sep. 30, 2015  | 
Sep. 30, 2014  | 
Sep. 30, 2015  | 
Sep. 30, 2014  | 
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| Loans At Acquisition Date [Line Items] | ||||
| Pro forma revenues (net interest income plus noninterest income) | $ 55,663 | $ 73,154 | $ 162,280 | $ 183,689 | 
| Pro forma net income from continuing operations | $ 15,044 | $ 25,569 | $ 42,931 | $ 51,572 | 
| Pro forma earnings per share from continuing operations: | ||||
| Basic (in usd per share) | $ 0.47 | $ 0.81 | $ 1.35 | $ 1.63 | 
| Diluted (in usd per share) | $ 0.47 | $ 0.80 | $ 1.35 | $ 1.61 | 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Definition The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- References No definition available. 
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
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