Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Funds [Member] - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments		$ 638.1	$ 616.7
Alternative Investments, Unfunded Commitments		524.0	475.1
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	270.8	263.2
Alternative Investments, Unfunded Commitments		144.6	152.7
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[1]	233.5	223.6
Alternative Investments, Unfunded Commitments		141.6	152.6
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]	37.3	39.6
Alternative Investments, Unfunded Commitments		3.0	0.1
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[2]	168.0	149.3
Alternative Investments, Unfunded Commitments		209.2	166.8
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	199.3	204.2
Alternative Investments, Unfunded Commitments		170.2	155.6
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	168.9	173.8
Alternative Investments, Unfunded Commitments		145.2	130.6
Real Assets | Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	[3]	30.4	30.4
Alternative Investments, Unfunded Commitments		$ 25.0	$ 25.0

[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, 2020, 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, 34 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, 2020, the estimated remaining life of the investments that do not allow for redemptions is approximately 31 percent in the next 3 years, 23 percent during the period from 3 to 5 years, 44 percent during the period from 5 to 10 years, and 2 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, 2020, the estimated remaining life of the investments that do not allow for redemptions is approximately 1 percent in the next 3 years, 14 percent during period from 3 to 5 years and 85 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 generally on a one-quarter lag in accordance with our accounting policy. We estimate that we will recognize losses totaling approximately $30 million to $50 million, or approximately 5 to 8 percent of the carrying value of our portfolio, in the second quarter of 2020 as we receive financial statements from the partnerships for the first quarter of 2020 that will reflect the impact of current economic conditions. During the first quarter of 2020, U.S. equity markets were severely depressed as a result of COVID-19 and although our partnership investments are not directly correlated with those markets, their results were impacted.