Segment information (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue - external		$ 1,758	$ 1,670
Depreciation and amortization		197	205
Operating income (loss)		398	349
Adjusted EBITDA		595	554
Earnings (losses) from equity method investments		2	2
Capital expenditures		525	276
Property, plant and equipment		21,797		$ 21,548
Equity method investments		384		387
Total assets		31,396		31,309
Operating Segments [Member] | Network [Member]
Segment Reporting Information [Line Items]
Revenue - external		1,460	1,390
Revenue - intersegment		(1)
Depreciation and amortization		113	118
Operating income (loss)		336	312
Adjusted EBITDA		449	430
Earnings (losses) from equity method investments		4	3
Capital expenditures		267	206
Property, plant and equipment		13,168		13,032
Equity method investments		149		151
Total assets		20,998		20,753
Operating Segments [Member] | Renewables [Member]
Segment Reporting Information [Line Items]
Revenue - external		285	276
Revenue - intersegment		2	2
Depreciation and amortization		78	80
Operating income (loss)		56	49
Adjusted EBITDA		133	129
Earnings (losses) from equity method investments		(2)	(1)
Capital expenditures		257	70
Property, plant and equipment		8,133		8,015
Equity method investments		235		236
Total assets		9,996		9,884
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Revenue - external		14	3
Revenue - intersegment		11	9
Depreciation and amortization		6	7
Operating income (loss)		7	(10)
Adjusted EBITDA		14	(3)
Capital expenditures		1
Property, plant and equipment		496		501
Total assets		1,041		1,124
Corporate and Intercompany Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue - external	[1]	(1)	1
Revenue - intersegment	[1]	(12)	(11)
Operating income (loss)	[1]	(1)	(2)
Adjusted EBITDA	[1]	(1)	$ (2)
Total assets	[1]	$ (639)		$ (452)

[1] Does not represent a segment. It mainly includes Corporate and intersegment eliminations.