Other Material Contingencies - Additional Information (Detail)		1 Months Ended	9 Months Ended
	Mar. 12, 2014 building Person lawsuit	Jul. 31, 2007 Person lawsuit	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Guarantor Obligations [Line Items]
Number of person died in steam ruptured | Person		1
Number of suits pending against the company | lawsuit		90
Estimated accrued liability for suits			$ 50,000,000
Number of buildings destroyed by fire | building	2
Insurance receivable for suits			50,000,000
Number of people died in explosion and fire incident | Person	8
Number of people injured in explosion and fire incident | Person	50
Guarantee obligations maximum exposure			$ 2,576,000,000	$ 2,547,000,000
Ownership interest, percentage				46.00%
Estimated project cost percentage			175.00%
Manhattan Explosion and Fire
Guarantor Obligations [Line Items]
Number of suits pending against the company | lawsuit	70
Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure			$ 30,000,000
Indemnity agreements amount			25,000,000
Electric provider obligation to Public Utility Commission of Texas			5,000,000
Construction and Operation of Solar Energy Facilities
Guarantor Obligations [Line Items]
Guarantees issued			$ 3,000,000