Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships - USD ($) $ in Millions		Sep. 30, 2023	Dec. 31, 2022
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,280.9	$ 1,194.3
Alternative Investments, Unfunded Commitments		836.9	776.9
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		286.7	275.0
Alternative Investments, Unfunded Commitments		130.9	104.3
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	238.0	239.3
Alternative Investments, Unfunded Commitments		127.7	90.9
Private Credit | Initial 2 year lock on each new investment / Quarterly after 2 year lock with 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments			35.7
Alternative Investments, Unfunded Commitments			13.4
Private Credit | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		48.7
Alternative Investments, Unfunded Commitments		3.2
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		541.3	485.3
Alternative Investments, Unfunded Commitments		437.8	416.3
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	514.9	453.6
Alternative Investments, Unfunded Commitments		420.2	377.2
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		26.4	31.7
Alternative Investments, Unfunded Commitments		17.6	39.1
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		452.9	434.0
Alternative Investments, Unfunded Commitments		268.2	256.3
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	420.3	373.9
Alternative Investments, Unfunded Commitments		268.2	256.3
Real Assets | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		32.6	60.1
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, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 63 percent in the next 3 years, 26 percent during the period from 3 to 5 years, 9 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 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, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 37 percent in the next 3 years, 28 percent during the period from 3 to 5 years, 28 percent during the period from 5 to 10 years, and 7 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 September 30, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 23 percent in the next 3 years, 33 percent during period from 3 to 5 years, 42 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years.