Consolidated Statements of Changes in Shareholder' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2017	$ 1,571,701	$ 14,115	$ 369,124	$ 1,228,187	$ (39,725)
Balance (in shares) at Dec. 31, 2017		67,746,094
Accumulated other comprehensive lossadjustment, Tax Cuts and Jobs Act of 2017(Tax Reform Act)				8,524	(8,524)
Net income per consolidated statements of income	36,830			36,830
Other comprehensive income (loss), net of tax	(19,637)				(19,637)
Common stock dividends paid ($0.23 per share)	(15,660)			(15,660)
Shares withheld to pay taxes, long-term incentive plan	(1,380)	$ 23	(1,403)
Shares withheld to pay taxes, long-term incentive plan (in shares)		109,558
Repurchase and retirement of common stock	(2,502)	$ (17)	(2,485)
Repurchase and retirement of common stock (in shares)		(80,584)
Compensation expense, long-term incentive plan	785		785
Balance at Mar. 31, 2018	1,570,137	$ 14,121	366,021	1,257,881	(67,886)
Balance (in shares) at Mar. 31, 2018		67,775,068
Balance at Dec. 31, 2018	$ 1,591,453	$ 13,717	309,545	1,323,870	(55,679)
Balance (in shares) at Dec. 31, 2018	65,834,395	65,834,395
Net income per consolidated statements of income	$ 33,339			33,339
Other comprehensive income (loss), net of tax	14,764				14,764
Common stock dividends paid ($0.23 per share)	(15,033)			(15,033)
Shares withheld to pay taxes, long-term incentive plan	(1,604)	$ 25	(1,629)
Shares withheld to pay taxes, long-term incentive plan (in shares)		123,821
Repurchase and retirement of common stock	(36,893)	$ (243)	(36,650)
Repurchase and retirement of common stock (in shares)		(1,168,273)
Compensation expense, long-term incentive plan	1,002		1,002
Balance at Mar. 31, 2019	$ 1,587,028	$ 13,499	$ 272,268	$ 1,342,176	$ (40,915)
Balance (in shares) at Mar. 31, 2019	64,789,943	64,789,943