Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
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	$ 1,289.6	$ 935.0
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net	8,125.2	7,594.4
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net	8,550.0	9,430.3
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net	20,001.1	19,790.7
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net	37,965.9	37,750.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	853.8	658.0
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses	38,819.7	38,408.4
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain	2.6	0.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	180.7	128.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	387.0	372.1
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	754.7	678.1
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	1,325.0	1,179.3
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	13.2	10.1
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	1,338.2	1,189.4
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss	5.5	7.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	126.9	179.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	442.7	610.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	1,782.9	1,942.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	2,358.0	2,739.9
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	16.4	24.0
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	2,374.4	2,763.9
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Total	37,783.5	36,833.9
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	342.0	140.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	3,864.2	2,685.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	4,462.2	4,100.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	9,350.9	8,524.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	18,019.3	15,450.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	465.0	267.1
Debt Securities, Available-for-sale, Total	18,484.3	15,718.0
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	944.7	787.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	4,314.8	4,857.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	4,032.1	5,091.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	9,622.0	10,001.8
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	18,913.6	20,738.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	385.6	377.0
Debt Securities, Available-for-sale, Total	$ 19,299.2	$ 21,115.9