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	41,598			41,598
Other comprehensive loss, net of tax	(1,492)				(1,492)
Cash dividends declared	(21,297)			(21,297)
Distributions from DDFP	21		21					214	(214)
Purchases of treasury stock	(1,933)					(1,933)
Shares issued dividend reinvestment plan	729		44			685
Stock option exercises	1,065		(58)			1,123
Allocation of ESOP shares	1,390		92				1,298
Allocation of SAP shares	2,816		2,816
Allocation of stock options	128		128
Ending Balance at Jun. 30, 2015	1,167,124	832	998,096	485,577	(1,463)	(271,904)	(44,014)	(6,899)	6,899
Beginning balance at Mar. 31, 2015					3,735
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,796
Other comprehensive loss, net of tax	(5,198)				(5,198)
Ending Balance at Jun. 30, 2015	1,167,124	832	998,096	485,577	(1,463)	(271,904)	(44,014)	(6,899)	6,899
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	42,329
Other comprehensive loss, net of tax	9,664				9,664
Cash dividends declared	(23,222)			(23,222)
Distributions from DDFP	59		59					335	(335)
Purchases of treasury stock	(2,702)					(2,702)
Shares issued dividend reinvestment plan	751		95			656
Stock option exercises	2,630		37			2,593
Allocation of ESOP shares	1,530		186				1,344
Allocation of SAP shares	2,371		2,371
Allocation of stock options	88		88
Ending Balance at Jun. 30, 2016	1,229,563	832	1,003,646	526,820	7,118	(268,467)	(40,386)	(6,182)	6,182
Beginning balance at Mar. 31, 2016					4,169
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,356
Other comprehensive loss, net of tax	2,949				2,949
Ending Balance at Jun. 30, 2016	$ 1,229,563	$ 832	$ 1,003,646	$ 526,820	$ 7,118	$ (268,467)	$ (40,386)	$ (6,182)	$ 6,182