Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
		Jul. 25, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016
Dividends declared (in dollars per share)			$ 0.44	$ 0.43	$ 0.88		$ 0.86
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ (10,967)		$ 21,587
Net change in unrealized gain (loss)			$ 21,958	$ 45,475	33,369		166,566
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities			0	(43)	0		(112)
Gain on available for sale securities, net			(380)	(5,326)	(2,429)		(9,290)
Other comprehensive income (loss), before income taxes			21,578	40,106	30,940		157,164
Federal and state income taxes			8,393	15,583	12,009	[1]	61,119	[1]
Other comprehensive income (loss), net of income taxes			13,185	24,523	18,931		96,045
Balance, End of Period			7,964	117,632	7,964		117,632
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)					33,369		166,566
Reclassification adjustments included in earnings:
Interest revenue, Investment securities, Taxable securities							0
Gain on available for sale securities, net					(2,429)		(9,290)
Other comprehensive income (loss), before income taxes					30,940		157,276
Federal and state income taxes	[1]				12,009		61,163
Other comprehensive income (loss), net of income taxes					18,931		96,113
Balance, End of Period			9,844	119,397	9,844		119,397
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)					0		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					0		0
Federal and state income taxes	[1]				0		0
Other comprehensive income (loss), net of income taxes					0		0
Balance, End of Period			(1,880)	$ (1,765)	(1,880)		$ (1,765)
Subsequent Event [Member]
Dividends Payable, Date Declared		Jul. 25, 2017
Dividends declared (in dollars per share)		$ 0.44
Dividends Payable, Date to be Paid		Aug. 25, 2017
Dividends Payable, Date of Record		Aug. 11, 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			$ 3,200		$ 3,200

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