Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
		Jul. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019		Jun. 30, 2018		Dec. 31, 2018	Dec. 31, 2017
Dividends declared (in dollars per share)			$ 0.50	$ 0.45	$ 1.00		$ 0.90
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)			$ 135,417	$ (33,117)	228,156		(130,523)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net			(1,029)	762	(1,105)		1,052
Other comprehensive income (loss), before income taxes			134,388	(32,355)	227,051		(129,471)
Federal and state income taxes			32,288	(8,241)	55,897	[1]	(33,049)	[1]
Other comprehensive income (loss), net of income taxes			102,100	(24,114)	171,154		(96,422)
Balance, End of Period			98,569	(135,305)	98,569		(135,305)
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)					228,156		(130,523)
Reclassification adjustments included in earnings:
Loss (gain) on available for sale securities, net					(1,105)		1,052
Other comprehensive income (loss), before income taxes					227,051		(129,471)
Federal and state income taxes	[1]				55,897		(33,049)
Other comprehensive income (loss), net of income taxes					171,154		(96,422)
Balance, End of Period			100,155	(134,516)	100,155		(134,516)
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		Jul. 30, 2019
Dividends declared (in dollars per share)		$ 0.50
Dividends Payable, Date to be Paid		Aug. 27, 2019
Dividends Payable, Date of Record		Aug. 12, 2019

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