Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Unallocated ESOP Shares [Member]	Common Stock Acquired By Directors Deferred Fee Plan [Member]	Deferred Compensation DDFP [Member]
Beginning balance at Dec. 31, 2014	$ 1,144,099	$ 832	$ 995,053	$ 465,276	$ 29	$ (271,779)	$ (45,312)	$ (7,113)	$ 7,113
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,802			19,802
Other comprehensive income, net of tax	3,706				3,706
Cash dividends declared	(10,798)			(10,798)
Distributions from DDFP	0							23	(23)
Purchases of treasury stock	(1,882)					(1,882)
Shares issued dividend reinvestment plan	377		23			354
Stock option exercises	395		(17)			412
Allocation of ESOP shares	687		38				649
Allocation of SAP shares	1,213		1,213
Allocation of stock options	72		72
Ending Balance at Mar. 31, 2015	1,157,671	832	996,382	474,280	3,735	(272,895)	(44,663)	(7,090)	7,090
Beginning balance at Dec. 31, 2015	1,196,065	832	1,000,810	507,713	(2,546)	(269,014)	(41,730)	(6,517)	6,517
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,973			20,973
Other comprehensive income, net of tax	6,715				6,715
Cash dividends declared	(11,321)			(11,321)
Distributions from DDFP	30		30					167	(167)
Purchases of treasury stock	(2,697)					(2,697)
Shares issued dividend reinvestment plan	365		34			331
Stock option exercises	2,321		46			2,275
Allocation of ESOP shares	746		74				672
Allocation of SAP shares	879		879
Allocation of stock options	46		46
Ending Balance at Mar. 31, 2016	$ 1,214,122	$ 832	$ 1,001,919	$ 517,365	$ 4,169	$ (269,105)	$ (41,058)	$ (6,350)	$ 6,350