Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Funds - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,450.6	$ 1,326.2
Alternative Investments, Unfunded Commitments		768.5	803.1
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		289.2	283.6
Alternative Investments, Unfunded Commitments		129.2	136.8
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	236.9	239.1
Alternative Investments, Unfunded Commitments		118.9	128.2
Private Credit | Quarterly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		52.3	44.5
Alternative Investments, Unfunded Commitments		10.3	8.6
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		640.2	571.9
Alternative Investments, Unfunded Commitments		409.2	427.2
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	604.1	543.9
Alternative Investments, Unfunded Commitments		398.2	410.6
Private Equity Limited Partnership | Quarterly after 5.5 year lock with 90 days notice [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		36.1	28.0
Alternative Investments, Unfunded Commitments		11.0	16.6
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		521.2	470.7
Alternative Investments, Unfunded Commitments		230.1	239.1
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	486.6	437.5
Alternative Investments, Unfunded Commitments		230.1	239.1
Real Assets | Quarterly [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		34.6	33.2
Alternative Investments, Unfunded Commitments		$ 0.0	$ 0.0

[1] Private Credit - The limited partnerships described in this category employ various investment strategies, generally providing direct lending or other forms of debt financing including first-lien, second-lien, mezzanine, and subordinated loans. The limited partnerships have credit exposure to corporates, physical assets, and/or financial assets within a variety of industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail) in North America and, to a lesser extent, outside of North America.  As of December 31, 2024, the estimated remaining life of the investments that do not allow for redemptions is approximately 78 percent in the next 3 years, 12 percent during the period from 3 to 5 years, and 10 percent during the period from 5 to 10 years.

[2] Private Equity - The limited partnerships described in this category employ various strategies generally investing in controlling or minority control equity positions directly in companies and/or assets across various industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail), primarily in private markets within North America and, to a lesser extent, outside of North America.  As of December 31, 2024, the estimated remaining life of the investments that do not allow for redemptions is approximately 36 percent in the next 3 years, 27 percent during the period from 3 to 5 years, 36 percent during the period from 5 to 10 years, and 1 percent during the period from 10 to 15 years.

[3] Real Assets - The limited partnerships described in this category employ various strategies, which include investing in the equity and/or debt financing of physical assets, including infrastructure (energy, power, water/wastewater, communications), transportation (including airports, ports, toll roads, aircraft, railcars) and real estate in North America, Europe, South America, and Asia.  As of December 31, 2024, the estimated remaining life of the investments that do not allow for redemptions is approximately 36 percent in the next 3 years, 26 percent during the period from 3 to 5 years, and 38 percent during the period from 5 to 10 years.