Segment Information - Schedule of Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Total operating revenues	$ 387,346	$ 371,799	$ 1,412,425	$ 1,439,212	$ 1,900,991	$ 1,956,863
Segment net income (loss)	(2,864)	(4,305)	88,017	70,938	150,410	126,262
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	363,231	344,428	1,351,000	1,376,347	1,819,057	1,867,865
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	24,115	27,371	61,425	62,865	81,934	88,998
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	195,031	195,573	927,500	982,203	1,267,025	1,362,655
Segment net income (loss)	(11,939)	(11,389)	69,317	64,609	121,327	113,381
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	195,031	195,573	927,500	982,203	1,267,025	1,362,655
Natural Gas Operations [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	192,315	176,226	484,925	457,009	633,966	594,208
Segment net income (loss)	9,075	7,084	18,700	6,329	29,083	12,881
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	168,200	148,855	423,500	394,144	552,032	505,210
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 24,115	$ 27,371	$ 61,425	$ 62,865	$ 81,934	$ 88,998