Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																				0 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Mar. 31, 2014 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Mar. 31, 2013 Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]		Mar. 31, 2014 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Mar. 31, 2013 Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]		Mar. 31, 2014 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Mar. 31, 2013 Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]		Mar. 31, 2014 Accumulated Loss on Effective Cash Flow Hedges [Member]		Mar. 31, 2013 Accumulated Loss on Effective Cash Flow Hedges [Member]		Apr. 29, 2014 Dividend Declared [Member]
Dividends Payable, Date Declared																					Apr. 29, 2014
Dividends declared (in dollars per share)	$ 0.40		$ 0.38																		$ 0.40
Dividends Payable, Date to be Paid																					May 30, 2014
Dividends Payable, Date of Record																					May 16, 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)	54,613		(21,359)		54,615		(21,359)		0		0		(2)		0		0		0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(403)		(1,148)		0		0		(403)		(1,148)		0		0		0		0
Interest expense, Subordinated debentures	83		52		0		0		0		0		0		0		83		52
Net impairment losses recognized in earnings	0		247				247				0				0				0
Gain on available for sale securities, net	(1,240)		(4,855)		(1,240)		(4,855)		0		0		0		0		0		0
Other comprehensive income (loss) before income taxes	53,053		(27,063)		53,375		(25,967)		(403)		(1,148)		(2)		0		83		52
Federal and state income taxes	(20,635)	[1]	10,526	[1]	(20,762)	[1]	10,100	[1]	158	[1]	446	[1]	1	[1]	0	[1]	(32)	[1]	(20)	[1]
Other comprehensive income (loss), net of income taxes	32,418		(16,537)		32,613		(15,867)		(245)		(702)		(1)		0		51		32
Balance, End of Period	$ 6,795		$ 133,383		$ 9,438		$ 139,686		$ 873		$ 2,376		$ (3,312)		$ (8,296)		$ (204)		$ (383)

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