Investments - Allowance for Credit Losses (Details)	3 Months Ended
	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
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	4,600,000
Allowance for Credit Losses at End of Period	4,600,000
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
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	1,100,000
Allowance for Credit Losses at End of Period	1,100,000
Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
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	3,500,000
Allowance for Credit Losses at End of Period	$ 3,500,000