Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions		Sep. 30, 2025	Dec. 31, 2024
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		$ 1,432.4	$ 1,484.1
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net		6,736.2	7,688.2
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net		7,357.1	8,404.6
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net		18,191.9	19,740.8
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net		33,717.6	37,317.7
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	[1]	1,128.9	949.4
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses		34,846.5	38,267.1
Available for Sale Securities Unrealized Gain Position Gross Gain
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain		3.9	4.1
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain		157.1	123.5
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain		257.5	236.4
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain		428.5	307.8
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal		847.0	671.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	[1]	10.0	5.0
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain		857.0	676.8
Available for Sale Securities Unrealized Loss Position Gross Loss
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss		6.4	6.2
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss		99.4	196.6
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss		369.7	565.5
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss		2,020.5	2,508.5
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal		2,496.0	3,276.8
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	[1]	17.3	37.2
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal		2,513.3	3,314.0
Fair Value Maturity Distribution
Debt Securities, Available-for-sale, Total		33,190.2	35,629.9
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		459.6	432.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value		3,523.8	2,840.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value		3,651.7	3,486.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value		6,379.9	4,965.7
Available for sale Securities Debt Maturities Fair Value Gross Subtotal		14,015.0	11,725.0
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	[1]	783.1	431.4
Debt Securities, Available-for-sale, Total		14,798.1	12,156.4
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		970.3	1,049.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value		3,270.1	4,774.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value		3,593.2	4,589.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value		10,220.0	12,574.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal		18,053.6	22,987.7
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	[1]	338.5	485.8
Debt Securities, Available-for-sale, Total		$ 18,392.1	$ 23,473.5

[1] 1Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types