Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
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	$ 894.4	$ 767.3
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net	6,651.9	6,613.2
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net	10,860.0	10,614.3
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net	18,722.9	18,804.0
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net	37,129.2	36,798.8
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	536.4	587.9
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses	37,665.6	37,386.7
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	16.3	17.6
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	253.2	540.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	864.3	1,453.3
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	2,048.1	3,987.9
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	3,181.9	5,999.0
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	23.4	50.4
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	3,205.3	6,049.4
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	0.3	0.1
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	14.3	6.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	239.3	26.0
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	670.3	68.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	924.2	100.1
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	0.7	0.0
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	924.9	100.1
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total	39,946.0	43,336.0
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	880.1	756.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	6,063.1	7,050.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	7,695.0	10,905.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	14,153.0	20,778.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	28,791.2	39,489.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	456.4	609.0
Debt Securities, Available-for-sale, Total	29,247.6	40,098.9
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	30.3	28.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	827.7	96.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	3,790.0	1,136.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	5,947.7	1,945.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	10,595.7	3,207.8
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	102.7	29.3
Debt Securities, Available-for-sale, Total	$ 10,698.4	$ 3,237.1