Revenue - Contract Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jul. 01, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Roll Forward]
Balance, beginning of year	$ 29,055	$ 30,753	$ 23,441
Deferred revenue	29,055	29,055	23,441
Balance, end of period	$ 32,568	$ 29,055	$ 30,753