Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) $ in Millions	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fixed-maturity securities available for sale:
Corporate debt securities	$ 26.1	$ 19.2
Investment in Ceridian	993.4
Total	1,019.5	19.2
Level 1
Fixed-maturity securities available for sale:
Corporate debt securities	0.0	0.0
Investment in Ceridian	993.4
Total	993.4	0.0
Level 2
Fixed-maturity securities available for sale:
Corporate debt securities	0.0	0.0
Investment in Ceridian	0.0
Total	0.0	0.0
Level 3
Fixed-maturity securities available for sale:
Corporate debt securities	26.1	19.2
Investment in Ceridian	0.0
Total	$ 26.1	$ 19.2
Level 3 \| Discount Rate \| Minimum
Fixed-maturity securities available for sale:
Discount rate	0.140
Level 3 \| Discount Rate \| Maximum
Fixed-maturity securities available for sale:
Discount rate	0.145
Level 3 \| Discount Rate \| Weighted Average
Fixed-maturity securities available for sale:
Discount rate	0.145

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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