Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$ 505,396	$ 432,475	$ 1,778,220	$ 1,494,024	$ 2,405,903	$ 2,032,381
Segment net income (loss)	(4,734)	1,970	72,198	82,380	130,944	139,683
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	474,508	409,317	1,704,467	1,422,408	2,311,578	1,934,037
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	30,888	23,158	73,753	71,616	94,325	98,344
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	219,420	226,027	1,059,178	983,999	1,457,266	1,356,653
Segment net income (loss)	(18,939)	(11,452)	59,325	62,945	113,252	117,797
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	219,420	226,027	1,059,178	983,999	1,457,266	1,356,653
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	285,976	206,448	719,042	510,025	948,637	675,728
Segment net income (loss)	14,205	13,422	12,873	19,435	17,692	21,886
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	255,088	183,290	645,289	438,409	854,312	577,384
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 30,888	$ 23,158	$ 73,753	$ 71,616	$ 94,325	$ 98,344