Investments Distribution of the Maturity Dates for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 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	$ 958.1	$ 935.0
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five, Net	7,744.6	7,594.4
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10, Net	9,081.5	9,430.3
Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10, Net	19,863.6	19,790.7
Available for sale Securities Debt Maturities Amortized Cost Gross Subtotal, Net	37,647.8	37,750.4
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost, Net	735.4	658.0
Debt Securities, Available-for-sale, Amortized Cost, Net of Allowance for Credit Losses	38,383.2	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	0.9	0.9
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Gain Position Gross Gain	118.0	128.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Gain Position Gross Gain	296.7	372.1
Available For Sale Securities Debt Maturities After Ten Years Unrealized Gain Position Gross Gain	481.4	678.1
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Gain Gross Subtotal	897.0	1,179.3
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Gain Position Gross Gain	5.0	10.1
Available for sale Securities, Debt Maturities Unrealized Gain Position Gross Gain	902.0	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	9.7	7.5
Available For Sale Securities Debt Maturities After One Through Five Years Unrealized Loss Position Gross Loss	205.6	179.0
Available For Sale Securities Debt Maturities After Five Through Ten Years Unrealized Loss Position Gross Loss	642.5	610.8
Available For Sale Securities Debt Maturities After Ten Years Unrealized Loss Position Gross Loss	2,147.9	1,942.6
Available For Sale Securities Debt Maturities Unrealized Gain Position Gross Loss Gross Subtotal	3,005.7	2,739.9
Available For Sale Securities Debt Maturities Without Single Maturity Date Unrealized Loss Position Gross Loss	31.7	24.0
Available For Sale Securities Unrealized Loss Position Gross Loss Subtotal	3,037.4	2,763.9
Fair Value Maturity Distribution [Abstract]
Debt Securities, Available-for-sale, Total	36,247.8	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	166.7	140.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	2,720.4	2,685.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	3,679.1	4,100.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	7,080.5	8,524.4
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	13,646.7	15,450.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	221.1	267.1
Debt Securities, Available-for-sale, Total	13,867.8	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	782.6	787.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	4,936.6	4,857.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	5,056.6	5,091.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	11,116.6	10,001.8
Available for sale Securities Debt Maturities Fair Value Gross Subtotal	21,892.4	20,738.9
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	487.6	377.0
Debt Securities, Available-for-sale, Total	$ 22,380.0	$ 21,115.9