Summary of Significant Accounting Policies - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Contract Assets
Balance at beginning of period	$ 10,213	$ 30,116	$ 30,116
Change due to acquisitions / disposals	6,573		2,833
Change due to conditional versus unconditional	(10,665)		6,244
Reclassified to unbilled accounts receivable			(28,980)
Balance at end of period	6,121		10,213
Contract Liabilities
Balance at beginning of period	130,986	106,005	106,005
Change due to acquisitions / disposals	31,556		8,195
Change in timing for performance obligation to be satisfied	4,555		16,786
Balance at end of period	167,097		$ 130,986
Revenue related to our contract liabilities	123,300
Impairment losses	$ 0	0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Contract Liabilities
Revenue related to our contract liabilities		$ 99,800