Supplementary Financial Information - Schedule of Financial Instruments Measured on Not Measured at Fair Value (Details) - Nonrecurring - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Level 1
Assets
Other invested assets	$ 0.0	$ 0.0
Total assets not accounted for at fair value	0.0	0.0
Liabilities
Debt obligations	0.0	0.0
Total liabilities not accounted for at fair value	0.0	0.0
Level 2
Assets
Other invested assets	0.0	0.0
Total assets not accounted for at fair value	0.0	0.0
Liabilities
Debt obligations	689.4	579.2
Total liabilities not accounted for at fair value	689.4	579.2
Level 3
Assets
Other invested assets	11.3	11.3
Total assets not accounted for at fair value	11.3	11.3
Liabilities
Debt obligations	0.0	0.0
Total liabilities not accounted for at fair value	0.0	0.0
Carrying Value
Assets
Other invested assets	11.3	11.3
Total assets not accounted for at fair value	11.3	11.3
Liabilities
Debt obligations	673.9	560.7
Total liabilities not accounted for at fair value	673.9	560.7
Estimated Fair Value
Assets
Other invested assets	11.3	11.3
Total assets not accounted for at fair value	11.3	11.3
Liabilities
Debt obligations	689.4	579.2
Total liabilities not accounted for at fair value	$ 689.4	$ 579.2