Investments - Allowance for Credit Losses (Details) - USD ($)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	$ 3,300,000	$ 0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2,200,000	5,400,000
End of the Period	5,100,000	5,400,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(200,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	(200,000)
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	300,000	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0	600,000
End of the Period	0	600,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(300,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	0
Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	3,000,000.0	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2,200,000	4,800,000
End of the Period	5,100,000	$ 4,800,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	100,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	$ (200,000)