Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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	$ 1,073.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,286.2	6,267.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	13,570.8	12,573.4
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	19,023.0	18,837.2
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal	39,701.5	38,751.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,377.8	1,523.8
Debt Securities, Available-for-sale, Amortized Cost, Total	41,079.3	40,275.2
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	14.5	14.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	456.5	300.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	1,688.3	795.0
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	4,195.0	2,315.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	6,354.3	3,425.7
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	101.3	67.2
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	6,455.6	3,492.9
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	0.2	8.4
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	41.7	80.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	14.6	303.9
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	34.3	355.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	90.8	748.1
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	0.4	8.3
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	91.2	756.4
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total	47,443.7	43,011.7
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	832.6	1,020.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	6,423.4	5,186.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	14,881.3	6,812.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	22,152.6	15,779.7
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	44,289.9	28,799.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,428.6	1,299.6
Debt Securities, Available-for-sale, Total	45,718.5	30,099.0
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	3.2	59.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	277.6	1,301.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	363.2	6,251.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	1,031.1	5,017.5
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	1,675.1	12,629.6
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	50.1	283.1
Debt Securities, Available-for-sale, Total	$ 1,725.2	$ 12,912.7