Fair Value Measurements (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and investments		$ 0
Carrying value
Fair Value of Financial Instruments [Abstract]
Notes	$ 864,500,000	864,500,000
Fair Value | Level 2
Fair Value of Financial Instruments [Abstract]
Notes	$ 910,000,000	$ 901,200,000