Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2018	Dec. 31, 2017
Dividends declared (in dollars per share)		$ 0.50	$ 0.45
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		$ (72,585)	$ (36,174)
Transition adjustment for new accounting principle in period of adoption					$ 2,862	$ (2,709)
Net change in unrealized gain (loss)		92,739	(97,406)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net		(76)	290
Other comprehensive income (loss), before income taxes		92,663	(97,116)
Federal and state income taxes		23,609	(24,808)	[1]
Other comprehensive income (loss), net of income taxes		69,054	(72,308)
Balance, End of Period		(3,531)	(111,191)
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		(70,999)	(35,385)
Transition adjustment for new accounting principle in period of adoption						(2,709)
Net change in unrealized gain (loss)		92,739	(97,406)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net		(76)	290
Other comprehensive income (loss), before income taxes		92,663	(97,116)
Federal and state income taxes		23,609	(24,808)	[1]
Other comprehensive income (loss), net of income taxes		69,054	(72,308)
Balance, End of Period		(1,945)	(110,402)
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period		(1,586)	(789)
Transition adjustment for new accounting principle in period of adoption						$ 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	0	[1]
Other comprehensive income (loss), net of income taxes		0	0
Balance, End of Period		$ (1,586)	$ (789)
Subsequent Event [Member]
Dividends Payable, Date Declared	Apr. 30, 2019
Dividends declared (in dollars per share)	$ 0.50
Dividends Payable, Date to be Paid	May 28, 2019
Dividends Payable, Date of Record	May 13, 2019

[1] Calculated using a 25 percent blended federal and state statutory tax rate.