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, 2012	$ 236,375	$ 311,909	$ (8,646)	$ 909,414	$ 1,449,052
Balance (in shares) at Dec. 31, 2012	94,549,867
Net income				94,115	94,115
Change in fair value of available-for-sale securities, net of tax effect			(38,065)		(38,065)
Change in pension funding status, net of tax effect			16,752		16,752
Comprehensive income					72,802
Exercise of stock options	$ 147	765			$ 912
Exercise of stock options (in shares)	58,982				64,000
Income tax benefit (expense) from exercise of stock options		139			$ 139
Recognition of stock compensation	$ 1,557	87			1,644
Recognition of stock compensation (in shares)	622,842
Cash dividends declared				(11,419)	(11,419)
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
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
Issuance of stock (in shares)	520,538
Exercise of stock options	$ 1,301	9,140			10,441
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