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Fair Value Measurements Fair Value Measurements Using Unobservable Inputs (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Balance, beginning of period $ 0 $ 0
Option to acquire tangible assets 6,497 0
Balance, end of period $ 6,497 $ 0
Option to acquire tangible asset, period exercisable 2 years