Accounts Receivable and Revenues from Contracts with Customers - Additional Information (Details) - Accounts Receivable $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) Customer	Jun. 30, 2018 USD ($) Customer	Dec. 31, 2018 USD ($)
Credit Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	67.00%		59.00%
Accounts receivable, major customers	$ 32.4		$ 34.8
Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of major customers | Customer	3	3
Revenues, major customers	$ 150.0	$ (157.8)
Concentration risk, percentage	66.00%	84.00%
Customer Concentration Risk [Member] | Major Customer 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ (85.6)
Concentration risk, percentage	38.00%
Customer Concentration Risk [Member] | Major Customer 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ (37.5)
Concentration risk, percentage	16.00%
Customer Concentration Risk [Member] | Major Customer 3
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ (26.9)
Concentration risk, percentage	12.00%