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, 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			105,630
Other comprehensive income (loss), net of tax	6,073				6,073
Cash dividends paid on common stock ($0.92 per share)	(62,795)			(62,795)
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		4,309
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	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 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
Net income per consolidated statements of income	$ 150,460			150,460
Other comprehensive income (loss), net of tax	32,079				32,079
Cash dividends paid on common stock ($0.92 per share)	(59,804)			(59,804)
Shares withheld to pay taxes, long-term incentive plan	(1,658)	$ 28	(1,686)
Shares withheld to pay taxes, long-term incentive plan (in shares)		134,564
Repurchase and retirement of common stock	(56,615)	$ (369)	(56,246)
Repurchase and retirement of common stock (in shares)		(1,768,848)
Compensation expense, long-term incentive plan	4,787		4,787
Balance at Dec. 31, 2019	$ 1,660,702	$ 13,376	$ 256,400	$ 1,414,526	$ (23,600)
Balance (in shares) at Dec. 31, 2019	64,200,111	64,200,111