Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended																				0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Sep. 30, 2012 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Sep. 30, 2013 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Sep. 30, 2012 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Sep. 30, 2013 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Sep. 30, 2012 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Sep. 30, 2013 Accumulated Loss on Effective Cash Flow Hedges [Member]		Sep. 30, 2012 Accumulated Loss on Effective Cash Flow Hedges [Member]		Oct. 29, 2013 Dividend Declared [Member]
Dividends Payable, Date Declared																							Oct. 29, 2013
Dividends declared (in dollars per share)	$ 0.38	$ 0.38	$ 1.14		$ 1.09																		$ 0.40
Dividends Payable, Date to be Paid																							Nov. 29, 2013
Dividends Payable, Date of Record																							Nov. 16, 2013
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			$ 149,920		$ 128,979		$ 155,553		$ 135,740		$ 3,078		$ 6,673		$ (8,296)		$ (12,742)		$ (415)		$ (692)
Net change in unrealized gain (loss)	(35,839)	46,064	(240,384)		86,098		(240,384)		86,390		0		0		0		(292)		0		0
Reclassification Adjustments Included in Earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(696)	(2,009)	(2,717)		(5,430)		0		0		(2,717)		(5,430)		0		0		0		0
Interest expense, Subordinated debentures	85	68	209		399		0		0		0		0		0		0		209		399
Net impairment losses recognized in earnings	1,509	1,104	2,308		5,684		2,308		5,684		0		0		0		0		0		0
Gain on available for sale securities, net	(478)	(7,967)	(9,086)		(32,779)		(9,086)		(32,779)		0		0		0		0		0		0
Other comprehensive income (loss), before income taxes	(35,419)	37,260	(249,670)		53,972		(247,162)		59,295		(2,717)		(5,430)		0		(292)		209		399
Income tax benefit (expense)	13,779	(14,057)	97,124	[1]	(20,558)	[1]	96,146	[1]	(23,066)	[1]	1,059	[1]	2,550	[1]	0	[1]	113	[1]	(81)	[1]	(155)	[1]
Other comprehensive income (loss), net of income taxes	(21,640)	23,203	(152,546)		33,414		(151,016)		36,229		(1,658)		(2,880)		0		(179)		128		244
Balance, End of Period	$ (2,626)	$ 162,393	$ (2,626)		$ 162,393		$ 4,537		$ 171,969		$ 1,420		$ 3,793		$ (8,296)		$ (12,921)		$ (287)		$ (448)

[1] Calculated using a 39% effective tax rate.