Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018		Mar. 31, 2017		Dec. 31, 2017
Dividends declared (in dollars per share)		$ 0.45		$ 0.44
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		$ (36,174)		$ (10,967)
Transition adjustment for unrealized gains and losses on equity securities		(2,709)
Net change in unrealized gain (loss)		(97,406)		11,411
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net		290		(2,049)
Other comprehensive income (loss), before income taxes		(97,116)		9,362
Federal and state income taxes		(24,808)	[1]	3,616	[2]
Other comprehensive income (loss), net of income taxes		(72,308)		5,746
Balance, End of Period		(111,191)		(5,221)
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		(35,385)		(9,087)
Transition adjustment for unrealized gains and losses on equity securities						$ (2,709)
Net change in unrealized gain (loss)		(97,406)		11,411
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net		290		(2,049)
Other comprehensive income (loss), before income taxes		(97,116)		9,362
Federal and state income taxes		(24,808)	[1]	3,616	[2]
Other comprehensive income (loss), net of income taxes		(72,308)		5,746
Balance, End of Period		(110,402)		(3,341)
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		(789)		(1,880)
Transition adjustment for unrealized gains and losses on equity securities		0
Net change in unrealized gain (loss)		0		0
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net		0		0
Other comprehensive income (loss), before income taxes		0		0
Federal and state income taxes		0	[1]	0	[2]
Other comprehensive income (loss), net of income taxes		0		0
Balance, End of Period		$ (789)		$ (1,880)
Subsequent Event [Member]
Dividends Payable, Date Declared	Apr. 24, 2018
Dividends declared (in dollars per share)	$ 0.45
Dividends Payable, Date to be Paid	May 25, 2018
Dividends Payable, Date of Record	May 11, 2018

[1] Calculated using a 25 percent effective tax rate.

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