Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
		Nov. 03, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Dividends declared (in dollars per share)			$ 0.51	$ 0.50	$ 1.53		$ 1.50
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period					$ 104,923		$ (72,585)
Net change in unrealized gain (loss)			$ (6,783)	$ 46,285	347,985		274,441
Reclassification Adjustments Included in Earnings [Abstract]
Loss (gain) on available for sale securities, net			12	(5)	(5,571)		(1,110)
Other comprehensive income, before income taxes			(6,771)	46,280	342,414		273,331
Federal and state income taxes			(1,625)	11,096	82,167	[1]	66,993	[1]
Other comprehensive income, net of income taxes			(5,146)	35,184	260,247		206,338
Balance, End of Period			365,170	133,753	365,170		133,753
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)					347,985		274,441
Reclassification Adjustments Included in Earnings [Abstract]
Loss (gain) on available for sale securities, net					(5,571)		(1,110)
Other comprehensive income, before income taxes					342,414		273,331
Federal and state income taxes	[1]				82,167		66,993
Other comprehensive income, net of income taxes					260,247		206,338
Balance, End of Period			365,243	135,339	365,243		135,339
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]
Loss (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		Nov. 03, 2020
Dividends declared (in dollars per share)		$ 0.52
Dividends Payable, Date to be Paid		Nov. 24, 2020
Dividends Payable, Date of Record		Nov. 16, 2020

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