Assets and Liabilities, at Fair Value - Contingent Consideration (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2015	Mar. 31, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in fair value of contingent consideration for acquisitions	$ 0.1
Business Combination, Contingent Consideration, Liability			$ 1.0
Contingent Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in fair value of contingent consideration for acquisitions		$ 0.2