Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
		Jul. 26, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016		Jun. 30, 2015
Dividends declared (in dollars per share)			$ 0.43	$ 0.42	$ 0.86		$ 0.84
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ 21,587		$ 56,673
Net change in unrealized gain (loss)			$ 45,475	$ (59,516)	166,566		(129)
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities			(43)	(134)	(112)		(313)
Interest expense, Subordinated debentures			0	56	0		121
Net impairment losses recognized in earnings			0	0	0		92
Gain on available for sale securities, net			(5,326)	(3,433)	(9,290)		(7,760)
Other comprehensive income (loss), before income taxes			40,106	(63,027)	157,164		(7,989)
Federal and state income taxes			15,583	(24,516)	61,119	[1]	(3,108)	[1]
Other comprehensive income (loss), net of income taxes			24,523	(38,511)	96,045		(4,881)
Balance, End of Period			117,632	51,792	117,632		51,792
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)					166,566		(129)
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					(9,290)		(7,760)
Other comprehensive income (loss), before income taxes					157,276		(7,797)
Federal and state income taxes	[1]				61,163		(3,033)
Other comprehensive income (loss), net of income taxes					96,113		(4,764)
Balance, End of Period			119,397	54,475	119,397		54,475
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					(112)		(313)
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					(112)		(313)
Federal and state income taxes	[1]				(44)		(122)
Other comprehensive income (loss), net of income taxes					(68)		(191)
Balance, End of Period			0	185	0		185
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)	(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		121
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		121
Federal and state income taxes	[1]				0		47
Other comprehensive income (loss), net of income taxes					0		74
Balance, End of Period			$ 0	$ 0	$ 0		$ 0
Subsequent Event [Member]
Dividends Payable, Date Declared		Jul. 26, 2016
Dividends declared (in dollars per share)		$ 0.43
Dividends Payable, Date to be Paid		Aug. 26, 2016
Dividends Payable, Date of Record		Aug. 12, 2016

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