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, 2014	$ 1,419,940	$ 14,060	$ 356,244	$ 1,092,120	$ (42,484)
Balance (in shares) at Dec. 31, 2014		67,481,992
Net income per consolidated statements of income	116,038	$ 0	0	116,038	0
Other comprehensive income (loss), net of tax	(2,910)				(2,910)
Cash dividends paid on common stock ($0.92 per share)	(62,605)	0	0	(62,605)	0
Common stock issued, long-term incentive plan	(1,074)	$ 16	1,555	(2,645)	0
Common stock issued, long-term incentive plan (in shares)		77,136
Compensation expense, long-term incentive plan	3,668	$ 0	3,668	0	0
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	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)
Common stock issued, long-term incentive plan	(1,724)	$ 24	(1,748)
Common stock issued, 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	67,746,094