Investments (Variable Interest Entities) (Details) $ in Millions	1 Months Ended
	Jan. 31, 2017 USD ($) Entity	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Variable Interest Entity [Line Items]
Number of special purpose entities | Entity	2
Other assets		$ 349.7	$ 0.0
Accounts payable and other liabilities		279.7	$ 0.0
Variable Interest Entity, Classification
Variable Interest Entity [Line Items]
Payments to acquire business	$ 70.0
Accounts receivable
Variable Interest Entity [Line Items]
Other assets		9.0
Investments
Variable Interest Entity [Line Items]
Other assets		340.7
Debt
Variable Interest Entity [Line Items]
Accounts payable and other liabilities		157.7
Obligations
Variable Interest Entity [Line Items]
Accounts payable and other liabilities		$ 122.0