Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$ 538,604	$ 453,153	$ 1,272,824	$ 1,061,549	$ 2,332,982	$ 1,987,252
Segment net income (loss)	4,949	9,627	76,932	80,410	137,648	134,849
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	515,755	428,445	1,229,959	1,013,091	2,246,387	1,887,951
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	22,849	24,708	42,865	48,458	86,595	99,301
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	286,643	271,479	839,758	757,972	1,463,873	1,325,657
Segment net income (loss)	(657)	1,798	78,264	74,397	120,739	117,310
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	286,643	271,479	839,758	757,972	1,463,873	1,325,657
Natural Gas Operations [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	251,961	181,674	433,066	303,577	869,109	661,595
Segment net income (loss)	5,606	7,829	(1,332)	6,013	16,909	17,539
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	229,112	156,966	390,201	255,119	782,514	562,294
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 22,849	$ 24,708	$ 42,865	$ 48,458	$ 86,595	$ 99,301