Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
		Apr. 26, 2016	Mar. 31, 2016	Mar. 31, 2015
Dividends declared (in dollars per share)			$ 0.43	$ 0.42
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			$ 21,587	$ 56,673
Net change in unrealized gain (loss)			121,091	59,387
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities			(69)	(179)
Interest expense, Subordinated debentures			0	65
Net impairment losses recognized in earnings			0	92
Gain on available for sale securities, net			(3,964)	(4,327)
Other comprehensive income (loss), before income taxes			117,058	55,038
Federal and state income taxes	[1]		45,536	21,408
Other comprehensive income (loss), net of income taxes			71,522	33,630
Balance, End of Period			93,109	90,303
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			23,284	59,239
Net change in unrealized gain (loss)			121,091	59,387
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	92
Gain on available for sale securities, net			(3,964)	(4,327)
Other comprehensive income (loss), before income taxes			117,127	55,152
Federal and state income taxes	[1]		45,563	21,452
Other comprehensive income (loss), net of income taxes			71,564	33,700
Balance, End of Period			94,848	92,939
Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			68	376
Net change in unrealized gain (loss)			0	0
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities			(69)	(179)
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			(69)	(179)
Federal and state income taxes	[1]		(27)	(69)
Other comprehensive income (loss), net of income taxes			(42)	(110)
Balance, End of Period			26	266
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			(1,765)	(2,868)
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			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	0
Federal and state income taxes	[1]		0	0
Other comprehensive income (loss), net of income taxes			0	0
Balance, End of Period			(1,765)	(2,868)
Accumulated Loss on Effective Cash Flow Hedges [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			0	(74)
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			0	65
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	65
Federal and state income taxes	[1]		0	25
Other comprehensive income (loss), net of income taxes			0	40
Balance, End of Period			$ 0	$ (34)
Dividend Declared [Member]
Dividends Payable, Date Declared		Apr. 26, 2016
Dividends declared (in dollars per share)		$ 0.43
Dividends Payable, Date to be Paid		May 27, 2016
Dividends Payable, Date of Record		May 13, 2016

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