Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2013	$ 238,079	$ 312,900	$ (29,959)	$ 992,110	$ 1,513,130
Balance (in shares) at Dec. 31, 2013	95,231,691
Net income				116,750	116,750
Change in fair value of available-for-sale securities, net of tax effect			16,028		16,028
Change in pension funding status, net of tax effect			(29,755)		(29,755)
Comprehensive income					103,023
Exercise of stock options	$ 1,669	9,914			$ 11,583
Exercise of stock options (in shares)	667,739				662,721
Income tax benefit (expense) from exercise of stock options		1,856			$ 1,856
Recognition of stock compensation	$ 963	238			1,201
Recognition of stock compensation (in shares)	385,113
Repurchase of stock	$ (74)	(637)			(711)
Repurchase of stock (in shares)	(29,640)
Cash dividends declared				(24,023)	(24,023)
Balance at Dec. 31, 2014	$ 240,637	324,271	(43,686)	1,084,837	1,606,059
Balance (in shares) at Dec. 31, 2014	96,254,903
Net income				127,491	127,491
Change in fair value of available-for-sale securities, net of tax effect			(5,847)		(5,847)
Change in pension funding status, net of tax effect			7,708		7,708
Comprehensive income					129,352
Exercise of stock options	$ 1,301	9,140			10,441
Exercise of stock options (in shares)	520,538
Income tax benefit (expense) from exercise of stock options		1,079			1,079
Recognition of stock compensation	$ 958	11,393			12,351
Recognition of stock compensation (in shares)	383,215
Repurchase of stock	$ (7,489)	(62,949)			(70,438)
Repurchase of stock (in shares)	(2,995,928)
Cash dividends declared				(33,400)	(33,400)
Balance at Dec. 31, 2015	$ 235,407	282,934	(41,825)	1,178,928	1,655,444
Balance (in shares) at Dec. 31, 2015	94,162,728
Net income				132,728	132,728
Change in fair value of available-for-sale securities, net of tax effect			(7,897)		(7,897)
Change in pension funding status, net of tax effect			(1,215)		(1,215)
Comprehensive income					123,616
Issuance of stock (in shares)	146,396
Exercise of stock options	$ 366	2,407			2,773
Income tax benefit (expense) from exercise of stock options		1,484			1,484
Recognition of stock compensation	$ 1,062	6,035			7,097
Recognition of stock compensation (in shares)	424,871
Repurchase of stock	$ (2,593)	(21,568)			$ (24,161)
Repurchase of stock (in shares)	(1,037,308)				(988,060)
Cash dividends declared				(42,370)	$ (42,370)
Balance at Dec. 31, 2016	$ 234,242	$ 271,292	$ (50,937)	$ 1,269,286	$ 1,723,883
Balance (in shares) at Dec. 31, 2016	93,696,687