Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Debt securities, available for sale, allowance for credit losses	$ 9.4	$ 13.6
Debt securities, available for sale, amortized cost	49,564.9	54,141.0
Mortgage loans, allowance for credit loss	96.4	179.2
Policy loans, fair value under fair value option	3,487.5	3,454.2
Reinsurance recoverable, allowance for credit losses	12.3	12.6
Funds withheld payable under reinsurance treaties, fair value under fair value option	$ 3,659.9	3,626.5
Common stock, par value (in usd per share)	$ 0.01
Accumulated other comprehensive income, tax expense	$ 275.4	$ 765.9
Class A common stock
Common stock, authorized (in shares)	900,000,000	900,000,000
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, issued (in shares)	93,099,859	93,099,859
Common stock, outstanding (in shares)	93,099,859	93,099,859
Class B common stock
Common stock, authorized (in shares)	100,000,000	100,000,000
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, issued (in shares)	1,364,484	1,364,484
Common stock, outstanding (in shares)	1,364,484	1,364,484