Operating Information by Segment and Geographic Area - Total Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Segment reporting information
Total assets	$ 9,041,714	$ 8,913,637
Reportable segments | Energy & Chemicals
Segment reporting information
Total assets	1,497,100	1,525,100
Reportable segments | Mining, Industrial, Infrastructure & Power
Segment reporting information
Total assets	1,229,600	1,193,200
Reportable segments | Government
Segment reporting information
Total assets	882,600	948,200
Reportable segments | Diversified Services
Segment reporting information
Total assets	$ 1,865,800	$ 1,841,000