Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended																				0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Sep. 30, 2013 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Sep. 30, 2014 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Sep. 30, 2013 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Sep. 30, 2014 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Sep. 30, 2013 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Sep. 30, 2014 Accumulated Loss on Effective Cash Flow Hedges [Member]		Sep. 30, 2013 Accumulated Loss on Effective Cash Flow Hedges [Member]		Oct. 29, 2014 Dividend Declared [Member]
Dividends Payable, Date Declared																							Oct. 28, 2014
Dividends declared (in dollars per share)	$ 0.40	$ 0.38	$ 1.20		$ 1.14																		$ 0.42
Dividends Payable, Date to be Paid																							Dec. 01, 2014
Dividends Payable, Date of Record																							Nov. 14, 2014
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			$ (25,623)		$ 149,920		$ (23,175)		$ 155,553		$ 1,118		$ 3,078		$ (3,311)		$ (8,296)		$ (255)		$ (415)
Net change in unrealized gain (loss)	(42,399)	(35,839)	82,252		(240,384)		82,254		(240,384)		0		0		(2)		0		0		0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(273)	(696)	(1,009)		(2,717)		0		0		(1,009)		(2,717)		0		0		0		0
Interest expense, Subordinated debentures	52	85	206		209		0		0		0		0		0		0		206		209
Net impairment losses recognized in earnings	0	1,509	0		2,308				2,308				0				0				0
Gain on available for sale securities, net	(146)	(478)	(1,390)		(9,086)		(1,390)		(9,086)		0		0		0		0		0		0
Other comprehensive income (loss) before income taxes	(42,766)	(35,419)	80,059		(249,670)		80,864		(247,162)		(1,009)		(2,717)		(2)		0		206		209
Federal and state income taxes	16,645	13,779	(31,141)	[1]	97,124	[1]	(31,456)	[1]	96,146	[1]	394	[1]	1,059	[1]	1	[1]	0	[1]	(80)	[1]	(81)	[1]
Other comprehensive income (loss), net of income taxes	(26,121)	(21,640)	48,918		(152,546)		49,408		(151,016)		(615)		(1,658)		(1)		0		126		128
Balance, End of Period	$ 23,295	$ (2,626)	$ 23,295		$ (2,626)		$ 26,233		$ 4,537		$ 503		$ 1,420		$ (3,312)		$ (8,296)		$ (129)		$ (287)

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