Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - Interest Rate Swaps - Receive Fixed/Pay Fixed - Designated as Hedging Instrument - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Fixed Maturity Securities - at fair value (amortized cost of $38,224.3; $37,825.2; allowance for credit losses of $—; $—)	Fixed Maturity Securities - at fair value (amortized cost of $38,224.3; $37,825.2; allowance for credit losses of $—; $—)
Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge, Cumulative Increase	$ (33.5)	$ (24.8)