Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
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	$ 945.9	$ 1,133.5
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net	7,541.9	7,090.8
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net	9,393.8	10,096.7
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net	19,687.7	18,912.1
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net	37,569.3	37,233.1
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	655.0	592.1
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses	38,224.3	37,825.2
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	1.3	2.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	67.7	86.7
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	187.5	294.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	164.1	351.0
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	420.6	735.4
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	4.1	8.2
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	424.7	743.6
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	5.0	5.7
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	311.5	238.4
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	955.4	863.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	3,252.2	2,593.1
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	4,524.1	3,701.0
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	54.2	27.0
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	4,578.3	3,728.0
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Total	34,070.7	34,840.8
Available for Sale Securities Unrealized Gain Position Fair Value
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	352.8	339.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,988.1	1,953.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	2,879.0	3,538.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	2,880.9	5,013.2
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	8,100.8	10,844.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	114.1	186.6
Debt Securities, Available-for-sale, Total	8,214.9	11,031.0
Available for Sale Securities Unrealized Loss Position Fair Value
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	589.4	791.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	5,310.0	4,985.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	5,746.9	5,988.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	13,718.7	11,656.8
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	25,365.0	23,423.1
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	490.8	386.7
Debt Securities, Available-for-sale, Total	$ 25,855.8	$ 23,809.8