Segment Information - Schedule of Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 668,146	$ 593,153	$ 2,093,359	$ 1,808,359	$ 2,833,792	$ 2,449,884
Segment net income (loss)	12,331	10,204	112,973	97,376	209,438	162,556
Operating Segments
Segment Reporting Information [Line Items]
Revenues	631,698	564,909	1,996,681	1,735,896	2,712,418	2,354,503
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	36,448	28,244	96,678	72,463	121,374	95,381
Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Segment net income (loss)	(797)	(107)	(1,362)	(777)	(1,922)	(777)
Natural Gas Operations
Segment Reporting Information [Line Items]
Revenues	217,523	213,059	987,515	935,823	1,354,000	1,276,308
Segment net income (loss)	(13,670)	(4,024)	79,301	82,436	153,683	134,323
Natural Gas Operations | Operating Segments
Segment Reporting Information [Line Items]
Revenues	217,523	213,059	987,515	935,823	1,354,000	1,276,308
Natural Gas Operations | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Infrastructure Services
Segment Reporting Information [Line Items]
Revenues	450,623	380,094	1,105,844	872,536	1,479,792	1,173,576
Segment net income (loss)	26,798	14,335	35,034	15,717	57,677	29,010
Infrastructure Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	414,175	351,850	1,009,166	800,073	1,358,418	1,078,195
Infrastructure Services | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	$ 36,448	$ 28,244	$ 96,678	$ 72,463	$ 121,374	$ 95,381