Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2014	$ 267,820	$ 1,689,291	$ 723,496	$ (50,074)	$ (158,131)	$ 2,472,402
Balance, Shares at Dec. 31, 2014	80,426,485
Net income			40,159			40,159
Other comprehensive income				4,305		4,305
Comprehensive income			40,159	4,305		44,464
Cash dividends declared ($0.24 per common share)			(19,524)			(19,524)
Common stock activity, long-term incentive plan	$ 78	12,086			(9,138)	3,026
Common stock activity, long-term incentive plan, Shares	23,242
Purchase of common stock under stock buyback program	$ (8,537)				(66,733)	(75,270)
Purchase of common stock under stock buyback program, Shares	(2,563,607)
Balance at Mar. 31, 2015	$ 259,361	1,701,377	744,131	(45,769)	(234,002)	2,425,098
Balance, Shares at Mar. 31, 2015	77,886,120
Balance at Dec. 31, 2015	$ 258,063	1,684,101	777,944	(80,595)	(226,370)	$ 2,413,143
Balance, Shares at Dec. 31, 2015	77,496,429					77,496,429
Net income			3,839			$ 3,839
Other comprehensive income				19,529		19,529
Comprehensive income			3,839	19,529		23,368
Cash dividends declared ($0.24 per common share)			(19,131)			(19,131)
Common stock activity, long-term incentive plan	$ 39	4,381			(760)	3,660
Common stock activity, long-term incentive plan, Shares	11,531
Balance at Mar. 31, 2016	$ 258,102	$ 1,688,482	$ 762,652	$ (61,066)	$ (227,130)	$ 2,421,040
Balance, Shares at Mar. 31, 2016	77,507,960					77,507,960