Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
		Apr. 28, 2020	Mar. 31, 2020	Mar. 31, 2019
Dividends declared (in dollars per share)			$ 0.51	$ 0.50
Schedule of Accumulated Other Income (Loss) [Abstract]
Balance, Beginning of Period			$ 104,923	$ (72,585)
Net change in unrealized gain (loss)			297,843	92,739
Reclassification adjustments included in earnings:
Gain on available for sale securities, net			(3)	(76)
Other comprehensive income, before income taxes			297,840	92,663
Federal and state income taxes	[1]		71,471	23,609
Other comprehensive income, net of income taxes			226,369	69,054
Balance, End of Period			331,292	(3,531)
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)			297,843	92,739
Reclassification adjustments included in earnings:
Gain on available for sale securities, net			(3)	(76)
Other comprehensive income, before income taxes			297,840	92,663
Federal and state income taxes	[1]		71,471	23,609
Other comprehensive income, net of income taxes			226,369	69,054
Balance, End of Period			331,365	(1,945)
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:
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)
Subsequent Event [Member]
Dividends Payable, Date Declared		Apr. 28, 2020
Dividends declared (in dollars per share)		$ 0.51
Dividends Payable, Date to be Paid		May 27, 2020
Dividends Payable, Date of Record		May 11, 2020

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