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 Directors Deferred Fee Plan [Member]	Deferred Compensation DDFP [Member]
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 (loss), net of tax	(9,047)				(9,047)
Cash dividends paid	(13,638)			(13,638)
Effect of adopting Accounting Standards Update	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
Beginning balance at Dec. 31, 2018	1,358,980	832	1,021,533	651,099	(12,336)	(272,470)	(29,678)	(4,504)	4,504
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,890			30,890
Other comprehensive income (loss), net of tax	7,252				7,252
Cash dividends paid	(28,964)			(28,964)
Effect of adopting Accounting Standards Update	4,350			4,350	0
Distributions from DDFP	42		42					167	(167)
Purchases of treasury stock	(180)					(180)
Purchase of employee restricted shares to fund statutory tax withholding	(1,914)					(1,914)
Shares issued dividend reinvestment plan	840		307			533
Stock option exercises	15		(11)			26
Allocation of ESOP shares	1,075		370				705
Allocation of SAP shares	1,388		1,388
Allocation of stock options	42		42
Ending Balance at Mar. 31, 2019	$ 1,373,816	$ 832	$ 1,023,671	$ 657,375	$ (5,084)	$ (274,005)	$ (28,973)	$ (4,337)	$ 4,337