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 (Loss) Income [Member]	Treasury Stock [Member]	Unallocated ESOP Shares [Member]	Common Stock Acquired By DDFP [Member]	Deferred Compensation DDFP [Member]
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 income, net of tax	858				858
Cash dividends declared	(12,629)			(12,629)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0
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
Beginning balance at Dec. 31, 2017	1,298,661	832	1,012,908	586,132	(7,465)	(259,907)	(33,839)	(5,175)	5,175
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27,913			27,913
Other comprehensive income, net of tax	(9,047)				(9,047)
Cash dividends declared	(13,638)			(13,638)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0			184	(184)
Distributions from DDFP	40		40					167	(167)
Purchase of employee restricted shares to fund statutory tax withholding	(1,792)					(1,792)
Shares issued dividend reinvestment plan	419		135			284
Stock option exercises	175		(65)			240
Allocation of ESOP shares	1,085		380				705
Allocation of SAP shares	1,025		1,025
Allocation of stock options	45		45
Ending balance at Mar. 31, 2018	$ 1,304,886	$ 832	$ 1,014,468	$ 600,591	$ (16,696)	$ (261,175)	$ (33,134)	$ (5,008)	$ 5,008