Variable Interest Entities (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Assets [Member]
Variable Interest Entity [Line Items]
Variable interest entities, investments, carrying value	$ 33.5	$ 22.8
Accounts Payable and Accrued Liabilities [Member]
Variable Interest Entity [Line Items]
Variable interest entities, unfunded commitments	17.3	14.0
Other Investments [Member]
Variable Interest Entity [Line Items]
Variable interest entities, investments, carrying value	13.8	12.3
Unfunded Loan Commitment [Member]
Variable Interest Entity [Line Items]
Variable interest entities, total exposure of other investments including unfunded commitments	$ 22.9	$ 19.9