Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Dividends declared (in dollars per share)			$ 0.44	$ 0.43	$ 1.32		$ 1.29
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ (10,967)		$ 21,587
Net change in unrealized gain (loss)			$ 512	$ (33,458)	33,881		133,108
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities			0	0	0		(112)
Gain on available for sale securities, net			(2,487)	(2,394)	(4,916)		(11,684)
Other comprehensive income (loss), before income taxes			(1,975)	(35,852)	28,965		121,312
Federal and state income taxes			(768)	(13,947)	11,241	[1]	47,172	[1]
Other comprehensive income (loss), net of income taxes			(1,207)	(21,905)	17,724		74,140
Balance, End of Period			6,757	95,727	6,757		95,727
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					(9,087)		23,284
Net change in unrealized gain (loss)					33,876		133,108
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities							0
Gain on available for sale securities, net					(4,916)		(11,684)
Other comprehensive income (loss), before income taxes					28,960		121,424
Federal and state income taxes	[1]				11,239		47,216
Other comprehensive income (loss), net of income taxes					17,721		74,208
Balance, End of Period			8,634	97,492	8,634		97,492
Accumulated Unrealized Gain on Investment Securities Transferred from AFS [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					0		68
Net change in unrealized gain (loss)					0		0
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities							(112)
Gain on available for sale securities, net					0		0
Other comprehensive income (loss), before income taxes					0		(112)
Federal and state income taxes	[1]						(44)
Other comprehensive income (loss), net of income taxes					0		(68)
Balance, End of Period			0	0	0		0
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					(1,880)		(1,765)
Net change in unrealized gain (loss)					5		0
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities							0
Gain on available for sale securities, net					0		0
Other comprehensive income (loss), before income taxes					5		0
Federal and state income taxes	[1]				2		0
Other comprehensive income (loss), net of income taxes					3		0
Balance, End of Period			(1,877)	$ (1,765)	(1,877)		$ (1,765)
Subsequent Event [Member]
Dividends Payable, Date Declared		Oct. 31, 2017
Dividends declared (in dollars per share)		$ 0.45
Dividends Payable, Date to be Paid		Nov. 27, 2017
Dividends Payable, Date of Record		Nov. 13, 2017
Perpetual Preferred, Equity Securities and Mutual Funds [Member] | Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Reclassification adjustments included in earnings:
Balance, End of Period			$ 2,200		$ 2,200

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