Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 29, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018		Dec. 31, 2018	Dec. 31, 2017
Dividends declared (in dollars per share)			$ 0.50	$ 0.50	$ 1.50		$ 1.40
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)			$ 46,285	$ (35,941)	274,441		(166,464)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net			(5)	(250)	(1,110)		802
Other comprehensive income (loss), before income taxes			46,280	(36,191)	273,331		(165,662)
Federal and state income taxes			11,096	(9,134)	66,993	[1]	(42,183)	[1]
Other comprehensive income (loss), net of income taxes			35,184	(27,057)	206,338		(123,479)
Balance, End of Period			133,753	(162,362)	133,753		(162,362)
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)					274,441		(166,464)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net					(1,110)		802
Other comprehensive income (loss), before income taxes					273,331		(165,662)
Federal and state income taxes	[1]				66,993		(42,183)
Other comprehensive income (loss), net of income taxes					206,338		(123,479)
Balance, End of Period			135,339	(161,573)	135,339		(161,573)
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
Other comprehensive income (loss), net of income taxes					0		0
Balance, End of Period			$ (1,586)	$ (789)	$ (1,586)		$ (789)
Subsequent Event [Member]
Dividends Payable, Date Declared		Oct. 29, 2019
Dividends declared (in dollars per share)		$ 0.51
Dividends Payable, Date to be Paid		Nov. 27, 2019
Dividends Payable, Date of Record		Nov. 12, 2019

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