Tangible Equity Units (Detail) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2016 unit $ / unit	Jun. 30, 2015 USD ($) $ / unit shares
Debt Component of 7.50% Tangible Equity Units [Member] | Senior Notes [Member]
Tangible Equity Units [Line Items]
Tangible equity unit rate	7.50%
Principal amount per amortizing note	10.95
Debt instrument, stated interest rate	1.50%
Beginning date of quarterly cash installment payments	Sep. 07, 2015
Quarterly cash installment payment per amortizing note	0.9375
First quarterly cash installment payment per amortizing note	0.9063
Final Installment Payment Date	Jun. 07, 2018
7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Number of tangible equity units issued | shares		9.2
Tangible equity unit rate		7.50%
Stated amount per tangible equity unit		50.00
Total net proceeds | $		$ 445
Equity Component of 7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Purchase contract, mandatory settlement date	Jun. 07, 2018
Equity Component of 7.50% Tangible Equity Units [Member] | Settled In WGP Common Units [Member]
Tangible Equity Units [Line Items]
Maximum settlement rate | unit	0.8591
Minimum settlement rate | unit	0.7159