Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended																				0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Jun. 30, 2013 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Jun. 30, 2014 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Jun. 30, 2013 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Jun. 30, 2014 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Jun. 30, 2013 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Jun. 30, 2014 Accumulated Loss on Effective Cash Flow Hedges [Member]		Jun. 30, 2013 Accumulated Loss on Effective Cash Flow Hedges [Member]		Jul. 29, 2014 Dividend Declared [Member]
Dividends Payable, Date Declared																							Jul. 29, 2014
Dividends declared (in dollars per share)	$ 0.40	$ 0.38	$ 0.80		$ 0.76																		$ 0.40
Dividends Payable, Date to be Paid																							Aug. 29, 2014
Dividends Payable, Date of Record																							Aug. 15, 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)	70,038	(183,186)	124,651		(204,545)		124,653		(204,545)		0		0		(2)		0		0		0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(333)	(873)	(736)		(2,021)		0		0		(736)		(2,021)		0		0		0		0
Interest expense, Subordinated debentures	71	72	154		124		0		0		0		0		0		0		154		124
Net impairment losses recognized in earnings	0	552	0		799				799				0				0				0
Gain on available for sale securities, net	(4)	(3,753)	(1,244)		(8,608)		(1,244)		(8,608)		0		0		0		0		0		0
Other comprehensive income (loss) before income taxes	69,772	(187,188)	122,825		(214,251)		123,409		(212,354)		(736)		(2,021)		(2)		0		154		124
Federal and state income taxes	(27,151)	72,819	(47,786)	[1]	83,345	[1]	(48,013)	[1]	82,605	[1]	286	[1]	788	[1]	1	[1]	0	[1]	(60)	[1]	(48)	[1]
Other comprehensive income (loss), net of income taxes	42,621	(114,369)	75,039		(130,906)		75,396		(129,749)		(450)		(1,233)		(1)		0		94		76
Balance, End of Period	$ 49,416	$ 19,014	$ 49,416		$ 19,014		$ 52,221		$ 25,804		$ 668		$ 1,845		$ (3,312)		$ (8,296)		$ (161)		$ (339)

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