Fair Value Measurements (Details) - 1.5% Convertible Unsecured Senior Notes - Convertible Debt - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Jan. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount of the liability component	$ 200,000,000	$ 200,000,000	$ 200,000,000
Fair Value, Inputs, Level 1 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	$ 178,000,000	$ 166,000,000