Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Goodwill Policy
Number of operating segments	1	1	1
Number of reportable segments	1	1	1
Impairment of goodwill | $	$ 0.0	$ 0.0	$ 0.0