Segment Information - Schedule of Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 725,230	$ 668,146	$ 2,271,780	$ 2,093,359	$ 3,058,434	$ 2,833,792
Segment net income (loss)	5,353	12,331	122,218	112,973	191,522	209,438
Operating Segments
Segment Reporting Information [Line Items]
Revenues	690,876	631,698	2,149,671	1,996,681	2,897,089	2,712,418
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	34,354	36,448	122,109	96,678	161,345	121,374
Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Segment net income (loss)	(473)	(797)	(1,253)	(1,362)	(1,433)	(1,922)
Natural Gas Operations
Segment Reporting Information [Line Items]
Revenues	209,980	217,523	989,368	987,515	1,359,581	1,354,000
Segment net income (loss)	(20,012)	(13,670)	86,746	79,301	146,287	153,683
Natural Gas Operations | Operating Segments
Segment Reporting Information [Line Items]
Revenues	209,980	217,523	989,368	987,515	1,359,581	1,354,000
Natural Gas Operations | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Utility Infrastructure Services
Segment Reporting Information [Line Items]
Revenues	515,250	450,623	1,282,412	1,105,844	1,698,853	1,479,792
Segment net income (loss)	25,838	26,798	36,725	35,034	46,668	57,677
Utility Infrastructure Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	480,896	414,175	1,160,303	1,009,166	1,537,508	1,358,418
Utility Infrastructure Services | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	$ 34,354	$ 36,448	$ 122,109	$ 96,678	$ 161,345	$ 121,374