Tangible Equity Units - Additional Information (Detail) shares in Millions, $ in Millions		1 Months Ended	6 Months Ended
	Jun. 07, 2018 USD ($) $ / unit shares	Jun. 30, 2015 USD ($) $ / unit shares	Jun. 30, 2018 USD ($)
TEUs - senior amortizing notes due 2018 [Member]
Tangible Equity Units [Line Items]
Principal amount per amortizing note | $ / unit	10.95
Debt instrument, stated interest rate	1.50%
Final payment | $	$ 9		$ 17
Debt instrument, maturity date	Jun. 07, 2018
7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Number of tangible equity units issued		9.2
Tangible equity unit rate		7.50%
Stated amount per tangible equity unit | $ / unit		50.00
Total net proceeds | $		$ 445
Equity Component of 7.50% Tangible Equity Units [Member] | Settled in WGP Common Units [Member]
Tangible Equity Units [Line Items]
Purchase contract, mandatory settlement date	Jun. 07, 2018
Number of tangible equity units settled	9.2
Shares, Issued	8.2
Settlement rate	0.8921