Fair Values of Financial Instruments Private Equity Partnerships (Details) - Private Equity Partnerships $ in Millions	Jun. 30, 2018 USD ($)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	$ 446.2
Alternative Investments, Unfunded Commitments	378.7
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	182.7	[1]
Alternative Investments, Unfunded Commitments	98.3
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	158.1	[1]
Alternative Investments, Unfunded Commitments	87.9
Private Credit | Quarterly after 2 year lock with 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	24.6	[1]
Alternative Investments, Unfunded Commitments	10.4
Private Equity Limited Partnership | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	121.4	[2]
Alternative Investments, Unfunded Commitments	182.0
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	142.1	[3]
Alternative Investments, Unfunded Commitments	98.4
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	111.9	[3]
Alternative Investments, Unfunded Commitments	98.4
Real Assets | Quarterly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Alternative Investments	30.2	[3]
Alternative Investments, Unfunded Commitments	$ 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, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 25 percent in the next 3 years, 47 percent during the period from 3 to 5 years, 24 percent during the period from 5 to 10 years, and 4 percent after the period of 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 June 30, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 30 percent in the next 3 years, 29 percent during the period from 3 to 5 years, 39 percent during the period from 5 to 10 years, and 2 percent after the period of 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, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 2 percent in the next 3 years, 23 percent during period from 3 to 5 years, 71 percent during the period from 5 to 10 years, and 4 percent during the period from 10 to 15 years.