Reinsurance - Assets and Liabilities Held in Support of Reserves (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Total assets, net	$ 29,638.7	$ 31,859.9
Liabilities
Total liabilities	29,771.4	31,971.5
Derivatives related to funds withheld under reinsurance treaties
Liabilities
Net fair value, derivative asset (liability)	(233.2)	(839.7)
Funds withheld embedded derivative | Derivatives related to funds withheld under reinsurance treaties
Liabilities
Net fair value, derivative asset (liability)	(271.7)	(826.6)
Debt securities
Assets
Total assets, net	20,617.7	24,642.4
Equity securities
Assets
Total assets, net	128.5	42.2
Mortgage loans
Assets
Total assets, net	4,713.1	2,985.5
Policy Loans
Assets
Total assets, net	3,503.9	3,470.8
Derivative instruments, net
Assets
Total assets, net	38.4	(13.1)
Limited partnerships
Assets
Total assets, net	537.8	124.9
Cash and cash equivalents
Assets
Total assets, net	383.5	394.1
Accrued investment income
Assets
Total assets, net	171.4	190.3
Other assets and liabilities, net
Assets
Total assets, net	(455.6)	22.8
Funds held under reinsurance treaties
Liabilities
Total liabilities	$ 29,771.4	$ 31,971.5