Investments - Allowance for Credit Loss Aggregated By Investment Category (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ (28.0)	$ 0.0
For credit losses on securities for which losses were not previously recorded	(3.0)	(6.0)
For initial credit losses on purchased securities accounted for as PCD financial assets	0.0	(35.0)
(Additions) reductions in allowance recorded on previously impaired securities	9.0	9.0
For securities sold during the period	0.0	6.0
For securities intended/required to be sold prior to recovery of amortized cost basis	0.0	4.0
Writeoffs charged against the allowance	0.0	0.0
Balance at End of Period	(22.0)	(22.0)
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(2.0)	0.0
For credit losses on securities for which losses were not previously recorded	0.0	7.0
For initial credit losses on purchased securities accounted for as PCD financial assets	0.0	(9.0)
(Additions) reductions in allowance recorded on previously impaired securities	2.0	2.0
For securities sold during the period	0.0	0.0
For securities intended/required to be sold prior to recovery of amortized cost basis	0.0	0.0
Writeoffs charged against the allowance	0.0	0.0
Balance at End of Period	0.0	0.0
Corporates
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(21.0)	0.0
For credit losses on securities for which losses were not previously recorded	(3.0)	(15.0)
For initial credit losses on purchased securities accounted for as PCD financial assets	0.0	(16.0)
(Additions) reductions in allowance recorded on previously impaired securities	6.0	6.0
For securities sold during the period	0.0	3.0
For securities intended/required to be sold prior to recovery of amortized cost basis	0.0	4.0
Writeoffs charged against the allowance	0.0	0.0
Balance at End of Period	(18.0)	(18.0)
Hybrids
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0.0	0.0
For credit losses on securities for which losses were not previously recorded	0.0	0.0
For initial credit losses on purchased securities accounted for as PCD financial assets	0.0	(3.0)
(Additions) reductions in allowance recorded on previously impaired securities	0.0	0.0
For securities sold during the period	0.0	3.0
For securities intended/required to be sold prior to recovery of amortized cost basis	0.0	0.0
Writeoffs charged against the allowance	0.0	0.0
Balance at End of Period	0.0	0.0
Residential mortgage-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(5.0)	0.0
For credit losses on securities for which losses were not previously recorded	0.0	2.0
For initial credit losses on purchased securities accounted for as PCD financial assets	0.0	(7.0)
(Additions) reductions in allowance recorded on previously impaired securities	1.0	1.0
For securities sold during the period	0.0	0.0
For securities intended/required to be sold prior to recovery of amortized cost basis	0.0	0.0
Writeoffs charged against the allowance	0.0	0.0
Balance at End of Period	$ (4.0)	$ (4.0)