Tangible Equity Units - Settlement Rate Scenarios for Equity Component Table (Detail) - Equity Component of 7.5% Tangible Equity Units [Member]	Dec. 31, 2015 unit $ / shares
Threshold Appreciation Price [Member]
Tangible Equity Units [Line Items]
Equity component applicable market value | $ / shares	$ 69.8422
Reference Price [Member]
Tangible Equity Units [Line Items]
Equity component applicable market value | $ / shares	$ 58.20
Settled In WGP Shares [Member] | Exceeds Threshold Appreciation Price [Member]
Tangible Equity Units [Line Items]
Minimum settlement rate | unit	0.7159
Settled In WGP Shares [Member] | Less Than Reference Price [Member]
Tangible Equity Units [Line Items]
Maximum settlement rate | unit	0.8591