Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data	Common Stock [Member]	Capital Surplus [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2010
Issuance of stock	$ 27,381	$ 81,296			$ 108,677
Issuance of stock (in shares)	10,952,381
Balance at Dec. 31, 2011	208,709	227,567	(2,261)	828,897	1,262,912
Balance (in shares) at Dec. 31, 2011	83,483,796
Net income				84,295	84,295
Change in fair value of available-for-sale securities, net of tax effect			1,599		1,599
Change in pension funding status, net of tax effect			(7,984)		(7,984)
Comprehensive income					77,910
Exercise of stock options	17	67			84
Exercise of stock options (in shares)	6,533				6,333
Income tax benefit (expense) from exercise of stock options		(32)			(32)
Recognition of stock compensation	281	3,073			3,354
Recognition of stock compensation (in shares)	112,315
Repurchase of stock	(13)	(62)			(75)
Repurchase of stock (in shares)	(5,158)
Cash dividends declared				(3,778)	(3,778)
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
Issuance of stock					912
Exercise of stock options	147	765
Exercise of stock options (in shares)	58,982
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