Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships - USD ($) $ in Millions		Jun. 30, 2026	Dec. 31, 2025
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,425.1	$ 1,456.3
Alternative Investments, Unfunded Commitments		762.8	756.7
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		226.6	255.8
Alternative Investments, Unfunded Commitments		124.1	121.0
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	181.5	205.9
Alternative Investments, Unfunded Commitments		111.4	108.3
Private Credit | Quarterly/ 90 days notice [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		45.1	49.9
Alternative Investments, Unfunded Commitments		12.7	12.7
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		657.7	651.4
Alternative Investments, Unfunded Commitments		444.9	422.3
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	621.5	618.9
Alternative Investments, Unfunded Commitments		429.0	406.8
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		36.2	32.5
Alternative Investments, Unfunded Commitments		15.9	15.5
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		540.8	549.1
Alternative Investments, Unfunded Commitments		193.8	213.4
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	501.6	511.4
Alternative Investments, Unfunded Commitments		193.8	213.4
Real Assets | Quarterly/ 90 days notice [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		39.2	37.7
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 June 30, 2026, the estimated remaining life of the investments that do not allow for redemptions is approximately 57 percent in the next 3 years, 20 percent during the period from 3 to 5 years, and 23 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 June 30, 2026, the estimated remaining life of the investments that do not allow for redemptions is approximately 39 percent in the next 3 years, 8 percent during the period from 3 to 5 years, 52 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 June 30, 2026, the estimated remaining life of the investments that do not allow for redemptions is approximately 38 percent in the next 3 years, 23 percent during the period from 3 to 5 years, and 39 percent during the period from 5 to 10 years.