Basis of Presentation and Summary of Significant Accounting Policies (Details) - Blackstone - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Transactions with Affiliates
Expenses incurred for services from subsidiaries	$ 0.7	$ 0.1
Blackstone Capital Partners VI L.P.
Transactions with Affiliates
Beneficial ownership (shares)	6,896,548

Balance held at close of period in number of shares.

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