Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships - USD ($) $ in Millions		Sep. 30, 2025	Dec. 31, 2024
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,456.3	$ 1,450.6
Alternative Investments, Unfunded Commitments		745.4	768.5
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		262.5	289.2
Alternative Investments, Unfunded Commitments		125.4	129.2
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	211.2	236.9
Alternative Investments, Unfunded Commitments	[1]	112.7	118.9
Private Credit | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		51.3	52.3
Alternative Investments, Unfunded Commitments		12.7	10.3
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		654.7	640.2
Alternative Investments, Unfunded Commitments		410.9	409.2
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	622.8	604.1
Alternative Investments, Unfunded Commitments	[2]	395.8	398.2
Private Equity Limited Partnership | Initial 5.5 year lock on each new investment / Quarterly after 5.5 year lock with 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		31.9	36.1
Alternative Investments, Unfunded Commitments		15.1	11.0
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		539.1	521.2
Alternative Investments, Unfunded Commitments		209.1	230.1
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	501.5	486.6
Alternative Investments, Unfunded Commitments	[3]	209.1	230.1
Real Assets | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		37.6	34.6
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 September 30, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 73 percent in the next 3 years, 14 percent during the period from 3 to 5 years, and 13 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 September 30, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 41 percent in the next 3 years, 23 percent during the period from 3 to 5 years, and 36 percent during the period from 5 to 10 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 September 30, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 45 percent in the next 3 years, 34 percent during period from 3 to 5 years, and 21 percent during the period from 5 to 10 years.