Quarterly report pursuant to Section 13 or 15(d)

Acquisition - Business Acquisition, Unaudited Proforma Result of Operation Information (Detail)

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Acquisition - Business Acquisition, Unaudited Proforma Result of Operation Information (Detail) - Central Bancorp, Inc [Member] - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Loans At Acquisition Date [Line Items]        
Pro forma revenues (net interest income plus noninterest income) $ 53,491 $ 49,343 $ 107,022 $ 105,589
Pro forma net income from continuing operations $ 15,276 $ 8,521 $ 27,887 $ 24,801
Pro forma earnings per share from continuing operations:        
Basic (in usd per share) $ 0.48 $ 0.27 $ 0.88 $ 0.78
Diluted (in usd per share) $ 0.48 $ 0.27 $ 0.87 $ 0.78