Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
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	$ 1,073.3	$ 1,625.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	6,267.5	5,579.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	12,573.4	12,091.1
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	18,837.2	18,611.2
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal	38,751.4	37,907.3
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,523.8	1,873.2
Debt Securities, Available-for-sale, Amortized Cost	40,275.2	39,780.5
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	30.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	300.6	453.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	795.0	1,169.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	2,315.6	4,041.4
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	3,425.7	5,695.7
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	67.2	105.1
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	3,492.9	5,800.8
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	8.4	3.1
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	80.2	18.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	303.9	53.2
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	355.6	44.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	748.1	118.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	8.3	4.7
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	756.4	123.5
Fair Value Maturity Distribution [Abstract]
Fixed Maturity Securities	43,011.7	45,457.8
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	1,020.1	1,638.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	5,186.9	5,810.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	6,812.7	11,916.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	15,779.7	21,333.1
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	28,799.4	40,699.3
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,299.6	1,576.4
Fixed Maturity Securities	30,099.0	42,275.7
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	59.3	14.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,301.0	204.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	6,251.8	1,291.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	5,017.5	1,275.5
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	12,629.6	2,784.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	283.1	397.2
Fixed Maturity Securities	$ 12,912.7	$ 3,182.1