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	20,973			20,973
Other comprehensive loss, 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	(1,556)					(1,556)
Purchase of employee restricted shares to fund statutory tax withholding	(1,141)					(1,141)
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
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	23,508			23,508
Other comprehensive loss, net of tax	858				858
Cash dividends declared	(12,629)			(12,629)
Distributions from DDFP	57		57					167	(167)
Purchases of treasury stock	(443)					(443)
Purchase of employee restricted shares to fund statutory tax withholding	(703)					(703)
Shares issued dividend reinvestment plan	475		168			307
Stock option exercises	2,508		(1,015)			3,523
Allocation of ESOP shares	1,095		390				705
Allocation of SAP shares	541		541
Allocation of stock options	44		44
Ending Balance at Mar. 31, 2017	$ 1,267,092	$ 832	$ 1,005,962	$ 561,647	$ (2,539)	$ (261,537)	$ (37,273)	$ (5,679)	$ 5,679