Mergers and Acquisitions - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended	0 Months Ended	2 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Feb. 10, 2015	Mar. 31, 2015	Dec. 31, 2014	Feb. 10, 2015 Office
Business Acquisition [Line Items]
Loans	$ 700,800,000				$ 700,800,000
Goodwill increase	168,900,000
Net income	13,887,000	16,421,000	16,421,000
Unaudited proforma net interest income and non - interest income	78,200,000	76,100,000
Approximate unaudited proforma net income	22,200,000	19,800,000
Carrying value of goodwill	481,000,000				481,000,000
ESB Financial Corporation [Member]
Business Acquisition [Line Items]
Total Assets				1,900,000,000			1,900,000,000
Loans				700,849,000			700,849,000
Securities				486,891,000			486,891,000
Value of acquisition				338,969,000
Common shares issued				9,178,531
Cash consideration for outstanding shares				45,000,000
Goodwill increase				168,889,000
Purchase price allocation in core deposit intangible				5,346,000			5,346,000
Closing stock price				$ 32.00			32.00
Merger related expense	9,700,000					1,300,000
Total of net interest income and non-interest income					6,500,000
Net income					$ 2,700,000
Pennsylvania
Business Acquisition [Line Items]
Number of offices					38		16
Common Stock [Member]
Business Acquisition [Line Items]
Common shares issued	9,178,531
Common Stock [Member] | ESB Financial Corporation [Member]
Business Acquisition [Line Items]
Common shares issued				9,178,531