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Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) $ in Millions	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fixed-maturity securities available for sale:
Corporate debt securities	$ 34.5	$ 19.2
Total	1,551.1	19.2
Ceridian
Fixed-maturity securities available for sale:
Equity Securities:	1,333.9
CoreLogic
Fixed-maturity securities available for sale:
Equity Securities:	155.6
Black Knight, Inc.
Fixed-maturity securities available for sale:
Equity Securities:	26.0
Other
Fixed-maturity securities available for sale:
Equity Securities:	1.1
Level 1
Fixed-maturity securities available for sale:
Corporate debt securities	0.0	0.0
Total	1,490.6	0.0
Level 1 \| Ceridian
Fixed-maturity securities available for sale:
Equity Securities:	1,333.9
Level 1 \| CoreLogic
Fixed-maturity securities available for sale:
Equity Securities:	155.6
Level 1 \| Black Knight, Inc.
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 1 \| Other
Fixed-maturity securities available for sale:
Equity Securities:	1.1
Level 2
Fixed-maturity securities available for sale:
Corporate debt securities	0.0	0.0
Total	0.0	0.0
Level 2 \| Ceridian
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 2 \| CoreLogic
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 2 \| Black Knight, Inc.
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 2 \| Other
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 3
Fixed-maturity securities available for sale:
Corporate debt securities	34.5	19.2
Total	60.5	$ 19.2
Level 3 \| Ceridian
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 3 \| CoreLogic
Fixed-maturity securities available for sale:
Equity Securities:	0.0
Level 3 \| Black Knight, Inc.
Fixed-maturity securities available for sale:
Equity Securities:	26.0
Level 3 \| Other
Fixed-maturity securities available for sale:
Equity Securities:	$ 0.0
Level 3 \| Discount Rate \| Minimum
Fixed-maturity securities available for sale:
Discount rate	0.078
Level 3 \| Discount Rate \| Maximum
Fixed-maturity securities available for sale:
Discount rate	0.175
Level 3 \| Discount Rate \| Weighted Average
Fixed-maturity securities available for sale:
Discount rate	0.123