Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				0 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Apr. 28, 2015
Dividends declared (in dollars per share)	$ 0.42		$ 0.40
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period	$ 56,673		$ (25,623)
Net change in unrealized gain (loss)	59,387		54,613
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(179)		(403)
Interest expense, Subordinated debentures	65		83
Net impairment losses recognized in earnings	92		0
Gain on available for sale securities, net	(4,327)		(1,240)
Other comprehensive income (loss) before income taxes	55,038		53,053
Federal and state income taxes	21,408	[1]	20,635	[1]
Other comprehensive income (loss), net of income taxes	33,630		32,418
Balance, End of Period	90,303		6,795
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period	59,239		(23,175)
Net change in unrealized gain (loss)	59,387		54,615
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	0		0
Interest expense, Subordinated debentures	0		0
Net impairment losses recognized in earnings	92		0
Gain on available for sale securities, net	(4,327)		(1,240)
Other comprehensive income (loss) before income taxes	55,152		53,375
Federal and state income taxes	21,452	[1]	20,762	[1]
Other comprehensive income (loss), net of income taxes	33,700		32,613
Balance, End of Period	92,939		9,438
Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period	376		1,118
Net change in unrealized gain (loss)	0		0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	(179)		(403)
Interest expense, Subordinated debentures	0		0
Net impairment losses recognized in earnings	0		0
Gain on available for sale securities, net	0		0
Other comprehensive income (loss) before income taxes	(179)		(403)
Federal and state income taxes	(69)	[1]	(158)	[1]
Other comprehensive income (loss), net of income taxes	(110)		(245)
Balance, End of Period	266		873
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period	(2,868)		(3,311)
Net change in unrealized gain (loss)	0		(2)
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	0		0
Interest expense, Subordinated debentures	0		0
Net impairment losses recognized in earnings	0		0
Gain on available for sale securities, net	0		0
Other comprehensive income (loss) before income taxes	0		(2)
Federal and state income taxes	0	[1]	(1)	[1]
Other comprehensive income (loss), net of income taxes	0		(1)
Balance, End of Period	(2,868)		(3,312)
Accumulated Loss on Effective Cash Flow Hedges [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period	(74)		(255)
Net change in unrealized gain (loss)	0		0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities	0		0
Interest expense, Subordinated debentures	65		83
Net impairment losses recognized in earnings	0		0
Gain on available for sale securities, net	0		0
Other comprehensive income (loss) before income taxes	65		83
Federal and state income taxes	25	[1]	32	[1]
Other comprehensive income (loss), net of income taxes	40		51
Balance, End of Period	$ (34)		$ (204)
Dividend Declared [Member]
Dividends Payable, Date Declared					Apr. 28, 2015
Dividends declared (in dollars per share)					$ 0.42
Dividends Payable, Date to be Paid					May 29, 2015
Dividends Payable, Date of Record					May 15, 2015

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