Intangible Assets - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-lived Intangible Assets
Increase in cost of finite-lived intangible assets	$ 90.3
Amortization expense for finite-lived intangible assets	6.2	$ 0.6	$ 1.0
Expected aggregate annual amortization expense for finite-lived intangible assets in 2019	11.4
Expected aggregate annual amortization expense for finite-lived intangible assets in 2020	10.9
Expected aggregate annual amortization expense for finite-lived intangible assets in 2021	10.9
Expected aggregate annual amortization expense for finite-lived intangible assets in 2022	10.9
Expected aggregate annual amortization expense for finite-lived intangible assets in 2023	10.9
Renewable investment advisory contracts [Member]
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	204.1	73.9
Trade Name
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	$ 50.1	$ 1.9