Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses, Fiscal Year Maturity
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One, Net	$ 1,037.7
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net	6,811.8
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net	12,880.8
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net	19,192.0
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net	39,922.3
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	1,317.1
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses	41,239.4
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		$ 821.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost		6,286.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost		13,570.8
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost		19,023.0
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal		39,701.5
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost		1,377.8
Amortized Cost of Fixed Maturity Securities	41,239.4	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	17.8	14.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	629.0	456.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	2,189.8	1,688.3
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	5,225.3	4,195.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	8,061.9	6,354.3
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	115.2	101.3
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	8,177.1	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	2.1	0.2
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	52.9	41.7
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	45.7	14.6
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	45.4	34.3
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	146.1	90.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	0.2	0.4
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	146.3	91.2
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total	49,270.2	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	1,014.5	832.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	6,994.9	6,423.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	14,384.6	14,881.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	23,376.4	22,152.6
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	45,770.4	44,289.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,410.6	1,428.6
Debt Securities, Available-for-sale, Total	47,181.0	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	38.9	3.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	393.0	277.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	640.3	363.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	995.5	1,031.1
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	2,067.7	1,675.1
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	21.5	50.1
Debt Securities, Available-for-sale, Total	$ 2,089.2	$ 1,725.2