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			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		3,660			3,660
Balance at Mar. 31, 2016	$ 291,346	1,428,108	762,652	(61,066)	2,421,040
Balance, Shares Issued at Mar. 31, 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			49,014		$ 49,014
Other comprehensive income				2,020	2,020
Comprehensive income			49,014	2,020	51,034
Cash dividends declared ($0.24 per common share)			(20,793)		(20,793)
Common stock activity, long-term incentive plan		12,815	43		12,858
Issuance of stock from dividend reinvestment and stock purchase plan		755			755
Balance at Mar. 31, 2017	$ 291,358	$ 1,711,823	$ 878,953	$ (118,512)	$ 2,763,622
Balance, Shares Issued at Mar. 31, 2017	87,495				87,495,000