Investments - Impairment Losses (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Allowance for Credit Losses at Beginning of the Year	$ 0
Impact of Adopting ASU 2016-13	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	7,100,000
Reduction Due to Sales	(2,000,000.0)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	(400,000)
Write-offs Charged Against Allowance	(1,400,000)
Allowance for Credit Losses at End of Period	3,300,000
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Allowance for Credit Losses at Beginning of the Year	0
Impact of Adopting ASU 2016-13	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	1,200,000
Reduction Due to Sales	(700,000)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	(200,000)
Write-offs Charged Against Allowance	0
Allowance for Credit Losses at End of Period	300,000
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Allowance for Credit Losses at Beginning of the Year	0
Impact of Adopting ASU 2016-13	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	5,900,000
Reduction Due to Sales	(1,300,000)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	(200,000)
Write-offs Charged Against Allowance	(1,400,000)
Allowance for Credit Losses at End of Period	$ 3,000,000.0