Summary of Significant Accounting Policies - Goodwill (Detail)	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) Reporting_Unit	Jun. 30, 2018 USD ($) division	Dec. 31, 2017 USD ($) Reporting_Unit
Goodwill And Intangible Assets [Line Items]
Number of internal divisions | division		2
Number of reporting units impacted by impairment charges | Reporting_Unit	2
Non-cash charge for impairment of goodwill	$ 57,000,000	$ 0
Decrease in fair value of reporting units considered for impairment calculation	10.00%		10.00%
Goodwill	$ 1,868,600,000	1,897,664,000	$ 1,868,600,000
Intangible assets	263,179,000	261,820,000	263,179,000
Oil and Gas Infrastructure Services Division
Goodwill And Intangible Assets [Line Items]
Goodwill	$ 596,073,000	590,677,000	$ 596,073,000
Oil and Gas Infrastructure Division, Operating Units that have been Negatively Impacted by Various Factors
Goodwill And Intangible Assets [Line Items]
Number of reporting units with fair values that approximated carrying values after sensitivity of a decrease in fair value | Reporting_Unit			1
Goodwill		48,900,000
Intangible assets		$ 13,100,000