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, 2012	$ 1,287,369	$ 13,506	$ 285,905	$ 984,563	$ 3,395
Balance (in shares) at Dec. 31, 2012		64,820,414
Net income per consolidated statements of income	117,060	$ 0	0	117,060	0
Other comprehensive loss, net of tax	(47,126)	0	0	0	(47,126)
Cash dividends paid on common stock	(62,276)	$ 0	0	(62,276)	0
Common stock issued, long-term incentive plan (in shares)		307,696
Common stock issued, long-term incentive plan	2,832	$ 64	7,149	(4,381)	0
Common stock issued, business combination (in shares)		2,244,870
Common stock issued, business combination	53,495	$ 468	53,027	0	0
Compensation expense, long-term incentive plan	3,599	0	3,599	0	0
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 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	67,481,992
Net income per consolidated statements of income	$ 116,038			116,038
Other comprehensive loss, net of tax	(2,910)				(2,910)
Cash dividends paid on common stock	(62,605)			(62,605)
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)
Compensation expense, long-term incentive plan	3,668		3,668
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