Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships - USD ($) $ in Millions		Mar. 31, 2023	Dec. 31, 2022
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 1,228.8	$ 1,194.3
Alternative Investments, Unfunded Commitments		772.2	776.9
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	283.5	275.0
Alternative Investments, Unfunded Commitments		109.2	104.3
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	244.3	239.3
Alternative Investments, Unfunded Commitments		97.9	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	[1]		35.7
Alternative Investments, Unfunded Commitments		11.3	13.4
Private Credit | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	39.2
Private Equity Limited Partnership
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		499.7	485.3
Alternative Investments, Unfunded Commitments		421.0	416.3
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	477.5	453.6
Alternative Investments, Unfunded Commitments		401.6	377.2
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		22.2	31.7
Alternative Investments, Unfunded Commitments		19.4	39.1
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	445.6	434.0
Alternative Investments, Unfunded Commitments		242.0	256.3
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	384.7	373.9
Alternative Investments, Unfunded Commitments		242.0	256.3
Real Assets | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	60.9	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 March 31, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 66 percent in the next 3 years, 21 percent during the period from 3 to 5 years, 11 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 March 31, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 36 percent in the next 3 years, 17 percent during the period from 3 to 5 years, 40 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 March 31, 2023, the estimated remaining life of the investments that do not allow for redemptions is approximately 22 percent in the next 3 years, 36 percent during period from 3 to 5 years, and 42 percent during the period from 5 to 10 years.We record changes in our share of net asset value of the partnerships in net investment income. We receive financial information related to our investments in partnerships and generally record investment income on a one-quarter lag in accordance with our accounting policy.