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 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
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