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]	Total
Balance at Dec. 31, 2015	$ 291,346	$ 1,424,448	$ 777,944	$ (80,595)	$ 2,413,143
Balance, Shares issued at Dec. 31, 2015	87,491
Net income			50,746		50,746
Other comprehensive income				31,054	31,054
Comprehensive income			50,746	31,054	81,800
Cash dividends declared ($0.48 per common share)			(38,238)		(38,238)
Common stock activity, long-term incentive plan		6,624			6,624
Issuance of stock from dividend reinvestment and stock purchase plan		36			36
Balance at Jun. 30, 2016	$ 291,346	1,431,108	790,452	(49,541)	2,463,365
Balance, Shares Issued at Jun. 30, 2016	87,491
Balance at Dec. 31, 2016	$ 291,358	1,698,253	850,689	(120,532)	$ 2,719,768
Balance, Shares issued at Dec. 31, 2016	87,495				87,495,000
Net income			101,281		$ 101,281
Other comprehensive income				16,093	16,093
Comprehensive income			101,281	16,093	117,374
Cash dividends declared ($0.48 per common share)			(41,594)		(41,594)
Common stock activity, long-term incentive plan		16,801	83		16,884
Issuance of stock from dividend reinvestment and stock purchase plan		1,530			1,530
Balance at Jun. 30, 2017	$ 291,358	$ 1,716,584	$ 910,459	$ (104,439)	$ 2,813,962
Balance, Shares Issued at Jun. 30, 2017	87,495				87,495,000