Investments (Investments Classified by Contractual Maturity Date) (Details) $ in Millions	Mar. 31, 2019 USD ($)
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost	$ 249.2
Due after one year through five years, Amortized Cost	1,969.6
Due after five years through ten years, Amortized Cost	2,407.4
Due after ten years, Amortized Cost	269.4
Gross fixed maturities, Amortized Cost	4,895.6
Mortgage-backed and asset-backed securities, Amortized Cost	1,597.1
Total fixed maturities, Amortized Cost	6,492.7
Due in one year or less, Fair Value	250.9
Due after one year through five years, Fair Value	2,001.6
Due after five years through ten years, Fair Value	2,423.2
Due after ten years, Fair Value	278.5
Gross fixed maturities, Fair Value	4,954.2
Mortgage-backed and asset-backed securities, Fair Value	1,605.2
Total fixed maturities, Fair Value	$ 6,559.4

Available for sale securities debt securities including held for sale amortized cost.

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Available for sale securities debt securities including held for sale fair value.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth fiscal year through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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