Other Material Contingencies - Additional Information (Detail)		1 Months Ended	3 Months Ended
	Mar. 12, 2014 building Person lawsuit	Jul. 31, 2007 Person	Mar. 31, 2016 USD ($) lawsuit	Apr. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Guarantor Obligations [Line Items]
Number of person died in steam ruptured | Person		1
Number of suits pending against the company | lawsuit			80
Estimated accrued liability for suits			$ 50,000,000
Number of buildings destroyed by fire | building	2
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			$ 2,705,000,000		$ 2,856,000,000
Ownership interest, percentage						45.70%
Estimated project cost percentage			175.00%
Manhattan Explosion and Fire
Guarantor Obligations [Line Items]
Number of suits pending against the company | lawsuit	70
Financial And Performance Guarantee For CET Gas
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure			$ 25,000,000
Financial and Performance Guarantee, Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure			75,000,000
Indemnity agreements amount			70,000,000
Electric provider obligation to Public Utility Commission of Texas			$ 5,000,000
Subsequent Event | Financial and Performance Guarantee, Other
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure				$ 946,000,000
Stagecoach Gas Services LLC | Subsequent Event
Guarantor Obligations [Line Items]
Equity method ownership (percent)				50.00%