Mergers and Acquisitions - Additional Information (Detail) $ in Thousands	Aug. 04, 2015 USD ($)	May. 27, 2015 USD ($) Branch	Sep. 19, 2014 USD ($) shares	Feb. 15, 2014 USD ($) shares	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill recorded as a result of merger					$ 831,333	$ 832,213
OBA Financial Services, Inc. (OBA) [Member]
Business Acquisition [Line Items]
Assets acquired as a result of merger			$ 390,160
Loans acquired as a result of merger			291,393
Deposits acquired as a result of merger			295,922
Value of acquisition			$ 85,554
Common shares issued | shares			7,170,037
Common shares acquired | shares			4,025,895
Goodwill recorded as a result of merger			$ 20,107
Core deposit intangibles recorded as result of the acquisition			$ 4,304
BCSB Bancorp, Inc. (BCSB) [Member]
Business Acquisition [Line Items]
Assets acquired as a result of merger				$ 596,122
Loans acquired as a result of merger				304,932
Deposits acquired as a result of merger				532,197
Value of acquisition				$ 80,547
Common shares issued | shares				6,730,597
Common shares acquired | shares				3,235,961
Goodwill recorded as a result of merger				$ 42,451
Core deposit intangibles recorded as result of the acquisition				$ 6,591
Bank of America [Member]
Business Acquisition [Line Items]
Deposits acquired on acquisition		$ 280,000
Number of branch-banking locations acquired | Branch		5
Metro Bancorp Inc [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Assets acquired as a result of merger	$ 3,001,357
Value of acquisition	$ 474,000
Number of shares entitled to receive as per Merger agreement, Ratio	2.373