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Summary of Significant Accounting Policies - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Contract Assets      
Balance at beginning of period $ 10,213 $ 30,116 $ 30,116
Change due to acquisitions 27   2,833
Change due to conditional versus unconditional (1,954)   6,244
Reclassified to unbilled accounts receivable     (28,980)
Balance at end of period 8,286   10,213
Contract Liabilities      
Balance at beginning of period 130,986 106,005 106,005
Change due to acquisitions 44   8,195
Change in timing for performance obligation to be satisfied (10,740)   16,786
Balance at end of period 120,290   $ 130,986
Revenue related to our contract liabilities 105,600    
Impairment losses $ 0 0  
Calculated under Revenue Guidance in Effect before Topic 606 [Member]      
Contract Liabilities      
Revenue related to our contract liabilities   $ 85,500