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, 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			42,329
Other comprehensive income, 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	(1,557)					(1,557)
Purchase of employee restricted shares to fund statutory tax withholding	(1,145)					(1,145)
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 income, net of tax	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
Beginning balance at Dec. 31, 2016	1,251,781	832	1,005,777	550,768	(3,397)	(264,221)	(37,978)	(5,846)	5,846
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,891
Other comprehensive income, net of tax	2,120				2,120
Cash dividends declared	(25,309)			(25,309)
Distributions from DDFP	114		114					335	(335)
Purchases of treasury stock	(443)					(443)
Purchase of employee restricted shares to fund statutory tax withholding	(709)					(709)
Shares issued dividend reinvestment plan	910		284			626
Stock option exercises	2,515		(1,017)			3,532
Allocation of ESOP shares	2,120		710				1,410
Allocation of SAP shares	2,514		2,514
Allocation of stock options	97		97
Ending Balance at Jun. 30, 2017	1,283,601	832	1,008,479	573,350	(1,277)	(261,215)	(36,568)	(5,511)	5,511
Beginning balance at Mar. 31, 2017					(2,539)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24,383
Other comprehensive income, net of tax	1,262
Ending Balance at Jun. 30, 2017	$ 1,283,601	$ 832	$ 1,008,479	$ 573,350	$ (1,277)	$ (261,215)	$ (36,568)	$ (5,511)	$ 5,511