Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
		Aug. 04, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019
Dividends declared (in dollars per share)			$ 0.51	$ 0.50	$ 1.02		$ 1.00
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ 104,923		$ (72,585)
Net change in unrealized gain (loss)			$ 56,922	$ 135,417	354,765		228,156
Reclassification Adjustments Included in Earnings [Abstract]
Gain on available for sale securities, net			(5,577)	(1,029)	(5,580)		(1,105)
Other comprehensive income, before income taxes			51,345	134,388	349,185		227,051
Federal and state income taxes			12,321	32,288	83,792	[1]	55,897	[1]
Other comprehensive income, net of income taxes			39,024	102,100	265,393		171,154
Balance, End of Period			370,316	98,569	370,316		98,569
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					104,996		(70,999)
Net change in unrealized gain (loss)					354,765		228,156
Reclassification Adjustments Included in Earnings [Abstract]
Gain on available for sale securities, net					(5,580)		(1,105)
Other comprehensive income, before income taxes					349,185		227,051
Federal and state income taxes	[1]				83,792		55,897
Other comprehensive income, net of income taxes					265,393		171,154
Balance, End of Period			370,389	100,155	370,389		100,155
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member]
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					(73)		(1,586)
Net change in unrealized gain (loss)					0		0
Reclassification Adjustments Included in Earnings [Abstract]
Gain on available for sale securities, net					0		0
Other comprehensive income, before income taxes					0		0
Federal and state income taxes	[1]				0		0
Other comprehensive income, net of income taxes					0		0
Balance, End of Period			$ (73)	$ (1,586)	$ (73)		$ (1,586)
Subsequent Event [Member]
Dividends Payable, Date Declared		Aug. 04, 2020
Dividends declared (in dollars per share)		$ 0.51
Dividends Payable, Date to be Paid		Aug. 26, 2020
Dividends Payable, Date of Record		Aug. 17, 2020

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