Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost		$ 1,484.1	$ 935.0
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net		7,688.2	7,594.4
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net		8,404.6	9,430.3
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net		19,740.8	19,790.7
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net		37,317.7	37,750.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	[1]	949.4	658.0
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses		38,267.1	38,408.4
Available for Sale Securities Unrealized Gain Position Gross Gain [Abstract]
Available For Sale Securities Debt Maturities Within One Year Unrealized Gain Position Gross Gain		4.1	0.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain		123.5	128.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain		236.4	372.1
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain		307.8	678.1
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal		671.8	1,179.3
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	[1]	5.0	10.1
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain		676.8	1,189.4
Available for Sale Securities Unrealized Loss Position Gross Loss [Abstract]
Available For Sale Securities Debt Maturities Within One Year Unrealized Loss Position Gross Loss		6.2	7.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss		196.6	179.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss		565.5	610.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss		2,508.5	1,942.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal		3,276.8	2,739.9
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	[1]	37.2	24.0
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal		3,314.0	2,763.9
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total		35,629.9	36,833.9
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		432.4	140.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value		2,840.8	2,685.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value		3,486.1	4,100.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value		4,965.7	8,524.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal		11,725.0	15,450.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	[1]	431.4	267.1
Debt Securities, Available-for-sale, Total		12,156.4	15,718.0
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		1,049.6	787.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value		4,774.3	4,857.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value		4,589.4	5,091.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value		12,574.4	10,001.8
Available for sale Securities Debt Maturities Fair Value Gross Subtotal		22,987.7	20,738.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	[1]	485.8	377.0
Debt Securities, Available-for-sale, Total		$ 23,473.5	$ 21,115.9

[1] 2Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types