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, 2013	$ 1,354,953	$ 14,038	$ 349,680	$ 1,034,966	$ (43,731)
Balance (in shares) at Dec. 31, 2013		67,372,980
Net income per consolidated statements of income	123,562	$ 0	0	123,562	0
Other comprehensive income (loss), net of tax	1,247	0	0	0	1,247
Cash dividends paid on common stock	(62,474)	$ 0	0	(62,474)	0
Common stock issued, long-term incentive plan (in shares)		109,012
Common stock issued, long-term incentive plan	(1,500)	$ 22	2,412	(3,934)	0
Compensation expense, long-term incentive plan	4,152	0	4,152	0	0
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)	0	0	0	(2,910)
Cash dividends paid on common stock	(62,605)	$ 0	0	(62,605)	0
Common stock issued, long-term incentive plan (in shares)		77,136
Common stock issued, long-term incentive plan	(1,074)	$ 16	1,555	(2,645)	0
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	67,559,128
Net income per consolidated statements of income	$ 108,411			108,411
Other comprehensive income (loss), net of tax	(404)				(404)
Cash dividends paid on common stock	(62,666)			(62,666)
Common stock issued, long-term incentive plan (in shares)		103,112
Common stock issued, long-term incentive plan	(1,124)	$ 22	2,155	(3,301)
Repurchase and retirement of common stock (in shares)		(33,622)
Repurchase and retirement of common stock	(750)	$ (7)	(743)
Compensation expense, long-term incentive plan	3,684		3,684
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