Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 27, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014
Dividends declared (in dollars per share)			$ 0.42	$ 0.40	$ 1.26		$ 1.20
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ 56,673		$ (25,623)
Net change in unrealized gain (loss)			$ 57,892	$ (42,399)	57,763		82,252
Reclassification adjustments included in earnings [Abstract]
Interest revenue, Investment securities, Taxable securities			(105)	(273)	(418)		(1,009)
Interest expense, Subordinated debentures			0	52	121		206
Net impairment losses recognized in earnings			0	0	92		0
Gain on available for sale securities, net			(2,166)	(146)	(9,926)		(1,390)
Other comprehensive income (loss) before income taxes			55,621	(42,766)	47,632		80,059
Federal and state income taxes			21,637	(16,645)	18,529	[1]	31,141	[1]
Other comprehensive income (loss), net of income taxes			33,984	(26,121)	29,103		48,918
Balance, End of Period			85,776	23,295	85,776		23,295
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)					57,763		82,254
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					(9,926)		(1,390)
Other comprehensive income (loss) before income taxes					47,929		80,864
Federal and state income taxes	[1]				18,644		31,456
Other comprehensive income (loss), net of income taxes					29,285		49,408
Balance, End of Period			88,524	26,233	88,524		26,233
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					(418)		(1,009)
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					(418)		(1,009)
Federal and state income taxes	[1]				(162)		(394)
Other comprehensive income (loss), net of income taxes					(256)		(615)
Balance, End of Period			120	503	120		503
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	[1]				0		(1)
Other comprehensive income (loss), net of income taxes					0		(1)
Balance, End of Period			(2,868)	(3,312)	(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					121		206
Net impairment losses recognized in earnings					0		0
Gain on available for sale securities, net					0		0
Other comprehensive income (loss) before income taxes					121		206
Federal and state income taxes	[1]				47		80
Other comprehensive income (loss), net of income taxes					74		126
Balance, End of Period			$ 0	$ (129)	$ 0		$ (129)
Dividend Declared [Member]
Dividends Payable, Date Declared		Oct. 27, 2015
Dividends declared (in dollars per share)		$ 0.43
Dividends Payable, Date to be Paid		Nov. 27, 2015
Dividends Payable, Date of Record		Nov. 13, 2015

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