Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract Assets
Balance at beginning of period	$ 2,736	$ 10,213
Change due to acquisitions / disposals	9,509	6,573
Change related to credit allowance	(79)
Other changes in the period	6,456	(14,050)
Balance at end of period	18,622	2,736
Contract Liabilities
Balance at beginning of period	166,918	130,986
Change due to acquisitions / disposals	39,885	31,556
Other changes in the period	19,434	4,376
Balance at end of period	226,237	166,918
Revenue related to our contract liabilities	$ 165,800
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Contract Liabilities
Revenue related to our contract liabilities		$ 126,700