Revenue from Contracts with Customers (Contract Assets and Advance Billings) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018
Revenue from Contract with Customer [Abstract]
Contracts assets, current portion	$ 4,565	$ 5,075
Contracts assets, noncurrent portion	1,752	648
Advance billings, current portion	236,599	238,707
Advance billings, noncurrent portion	14,887	$ 17,874
Total increase (decrease)
Contracts assets, current portion	(510)
Contracts assets, noncurrent portion	1,104
Advance billings, current portion	(2,108)
Advance billings, noncurrent portion	$ (2,987)