Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 1,106.9	$ 821.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,322.2	6,286.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	13,288.5	13,570.8
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	18,996.0	19,023.0
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal	39,713.6	39,701.5
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,316.7	1,377.8
Debt Securities, Available-for-sale, Amortized Cost, Total	41,030.3	41,079.3
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	19.8	14.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	343.6	456.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	1,207.3	1,688.3
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	3,627.8	4,195.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	5,198.5	6,354.3
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	129.9	101.3
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	5,328.4	6,455.6
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	3.0	0.2
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	182.4	41.7
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	416.1	14.6
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	464.8	34.3
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	1,066.3	90.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	0.9	0.4
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	1,067.2	91.2
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total	45,291.5	47,443.7
Available for Sale Securities Unrealized Gain Position Fair Value
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	1,055.1	832.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	4,963.1	6,423.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	10,271.5	14,881.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	19,164.5	22,152.6
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	35,454.2	44,289.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,401.9	1,428.6
Debt Securities, Available-for-sale, Total	36,856.1	45,718.5
Available for Sale Securities Unrealized Loss Position Fair Value
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	68.6	3.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,520.3	277.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	3,808.2	363.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	2,994.5	1,031.1
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	8,391.6	1,675.1
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	43.8	50.1
Debt Securities, Available-for-sale, Total	$ 8,435.4	$ 1,725.2

Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain

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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).

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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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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 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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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 after 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 recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next 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), maturing in next 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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