Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships - USD ($) $ in Millions		Mar. 31, 2025	Dec. 31, 2024
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,435.3	$ 1,450.6
Alternative Investments, Unfunded Commitments		777.1	768.5
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		285.6	289.2
Alternative Investments, Unfunded Commitments		123.4	129.2
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	229.0	236.9
Alternative Investments, Unfunded Commitments		116.2	118.9	[1]
Private Credit | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		56.6	52.3
Alternative Investments, Unfunded Commitments		7.2	10.3
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		639.7	640.2
Alternative Investments, Unfunded Commitments		417.6	409.2
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	602.3	604.1
Alternative Investments, Unfunded Commitments		407.1	398.2	[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		37.4	36.1
Alternative Investments, Unfunded Commitments		10.5	11.0
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		510.0	521.2
Alternative Investments, Unfunded Commitments		236.1	230.1
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	474.1	486.6
Alternative Investments, Unfunded Commitments		236.1	230.1	[3]
Real Assets | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		35.9	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 March 31, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 75 percent in the next 3 years, 16 percent during the period from 3 to 5 years, and 9 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 March 31, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 34 percent in the next 3 years, 28 percent during the period from 3 to 5 years, 37 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 March 31, 2025, the estimated remaining life of the investments that do not allow for redemptions is approximately 48 percent in the next 3 years, 27 percent during the period from 3 to 5 years, and 25 percent during the period from 5 to 10 years.