Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2015	$ 1,473,057	$ 14,076	$ 361,467	$ 1,142,908	$ (45,394)
Balance (in shares) at Dec. 31, 2015		67,559,128
Net income per consolidated statements of income	108,411	$ 0	0	108,411	0
Other comprehensive income (loss), net of tax	(404)				(404)
Cash dividends paid on common stock ($0.92 per share)	(62,666)	0	0	(62,666)	0
Common stock issued, long-term incentive plan	(1,124)	$ 22	2,155	(3,301)	0
Common stock issued, long-term incentive plan (in shares)		103,112
Repurchase and retirement of common stock	(750)	$ (7)	(743)
Repurchase and retirement of common stock (in shares)		(33,622)
Compensation expense, long-term incentive plan	3,684	$ 0	3,684	0	0
Balance at Dec. 31, 2016	1,520,208	$ 14,091	366,563	1,185,352	(45,798)
Balance (in shares) at Dec. 31, 2016		67,628,618
Net income per consolidated statements of income	105,630	$ 0	0	105,630	0
Other comprehensive income (loss), net of tax	6,073				6,073
Cash dividends paid on common stock ($0.92 per share)	(62,795)	0	0	(62,795)	0
Shares withheld to pay taxes, long-term incentive plan	(1,724)	$ 24	(1,748)
Shares withheld to pay taxes, long-term incentive plan (in shares)		117,476
Compensation expense, long-term incentive plan	4,309	$ 0	4,309	0	0
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	67,746,094
Net income per consolidated statements of income	$ 149,584			149,584
Other comprehensive income (loss), net of tax	(7,430)				(7,430)
Cash dividends paid on common stock ($0.92 per share)	(62,425)			(62,425)
Shares withheld to pay taxes, long-term incentive plan	(1,426)	$ 25	(1,451)
Shares withheld to pay taxes, long-term incentive plan (in shares)		118,108
Accumulated other comprehensive loss adjustment, Tax Cuts and Jobs Act of2017 (Tax Reform Act)				8,524	(8,524)
Repurchase and retirement of common stock	(62,421)	$ (423)	(61,998)
Repurchase and retirement of common stock (in shares)		(2,029,807)
Compensation expense, long-term incentive plan	3,870		3,870
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