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	62,207			62,207
Other comprehensive loss, net of tax	2,069				2,069
Cash dividends declared	(31,810)			(31,810)
Distributions from DDFP	49		49					405	(405)
Purchases of treasury stock	0					0
Purchase of employee restricted shares to fund statutory tax withholding	(1,946)					(1,946)
Shares issued dividend reinvestment plan	1,069		75			994
Stock option exercises	2,054		(175)			2,229
Allocation of ESOP shares	2,119		171				1,948
Allocation of SAP shares	3,863		3,863
Allocation of stock options	200		200
Ending Balance at Sep. 30, 2015	1,183,973	832	999,236	495,673	2,098	(270,502)	(43,364)	(6,708)	6,708
Beginning balance at Jun. 30, 2015					(1,463)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,609
Other comprehensive loss, net of tax	3,561				3,561
Ending Balance at Sep. 30, 2015	1,183,973	832	999,236	495,673	2,098	(270,502)	(43,364)	(6,708)	6,708
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	65,235
Other comprehensive loss, net of tax	8,520				8,520
Cash dividends declared	(35,141)			(35,141)
Effect of adopting Accounting Standards Update (ASU) No. 2016-09	0		(622)	622
Distributions from DDFP	91		91					503	(503)
Purchases of treasury stock	(1,557)					(1,557)
Purchase of employee restricted shares to fund statutory tax withholding	(1,161)					(1,161)
Shares issued dividend reinvestment plan	1,176		180			996
Stock option exercises	5,598		(60)			5,658
Allocation of ESOP shares	2,341		325				2,016
Allocation of SAP shares	2,982		2,982
Allocation of stock options	131		131
Ending Balance at Sep. 30, 2016	1,244,280	832	1,003,837	538,429	5,974	(265,078)	(39,714)	(6,014)	6,014
Beginning balance at Jun. 30, 2016					7,118
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,906
Other comprehensive loss, net of tax	(1,144)				(1,144)
Ending Balance at Sep. 30, 2016	$ 1,244,280	$ 832	$ 1,003,837	$ 538,429	$ 5,974	$ (265,078)	$ (39,714)	$ (6,014)	$ 6,014