Consolidated Statements of Changes in Shareholders' 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, 2017	1,571,701	$ 14,115	369,124	1,228,187	(39,725)
Balance (in shares) at Dec. 31, 2017		67,746,094
Net income per consolidated statements of income	76,643
Other comprehensive income (loss), net of tax	(25,490)
Compensation expense, long-term incentive plan	1,864
Balance at Jun. 30, 2018	1,584,072	$ 14,089	361,715	1,282,007	(73,739)
Balance (in shares) at Jun. 30, 2018		67,621,111
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
Net income per consolidated statements of income	39,813			39,813
Other comprehensive income (loss), net of tax	(5,853)				(5,853)
Common stock dividends paid ($0.23 per share)	(15,687)			(15,687)
Shares withheld to pay taxes, long-term incentive plan	(42)	$ 1	(43)
Shares withheld to pay taxes, long-term incentive plan (in shares)		8,292
Repurchase and retirement of common stock	(5,375)	$ (33)	(5,342)
Repurchase and retirement of common stock (in shares)		(162,249)
Compensation expense, long-term incentive plan	1,079		1,079
Balance at Jun. 30, 2018	1,584,072	$ 14,089	361,715	1,282,007	(73,739)
Balance (in shares) at Jun. 30, 2018		67,621,111
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
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	$ 75,479
Other comprehensive income (loss), net of tax	30,863
Compensation expense, long-term incentive plan	2,281
Balance at Jun. 30, 2019	$ 1,618,550	$ 13,418	260,619	1,369,329	(24,816)
Balance (in shares) at Jun. 30, 2019	64,398,846	64,398,846
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
Net income per consolidated statements of income	42,140			42,140
Other comprehensive income (loss), net of tax	16,099				16,099
Common stock dividends paid ($0.23 per share)	(14,987)			(14,987)
Shares withheld to pay taxes, long-term incentive plan	(1)	$ 2	(3)
Shares withheld to pay taxes, long-term incentive plan (in shares)		7,087
Repurchase and retirement of common stock	(13,008)	$ (83)	(12,925)
Repurchase and retirement of common stock (in shares)		(398,184)
Compensation expense, long-term incentive plan	1,279		1,279
Balance at Jun. 30, 2019	$ 1,618,550	$ 13,418	$ 260,619	$ 1,369,329	$ (24,816)
Balance (in shares) at Jun. 30, 2019	64,398,846	64,398,846