Tangible Equity Units - Settlement Rate Scenarios for Equity Component Table (Detail) - Equity Component of 7.5% Tangible Equity Units [Member]	Dec. 31, 2017 unit $ / shares	[1]
Threshold Appreciation Price [Member]
Tangible Equity Units [Line Items]
Equity component applicable market value | $ / shares	$ 68.0643
Reference Price [Member]
Tangible Equity Units [Line Items]
Equity component applicable market value | $ / shares	$ 56.7083
Settled In WGP Shares [Member] | Exceeds Threshold Appreciation Price [Member]
Tangible Equity Units [Line Items]
Minimum settlement rate | unit	0.7346
Settled In WGP Shares [Member] | Less Than Reference Price [Member]
Tangible Equity Units [Line Items]
Maximum settlement rate | unit	0.8817

[1] The applicable market value is the average of the daily volume-weighted average prices of WGP common units (or APC Shares) for the 20 consecutive trading days beginning on, and including, the 23rd scheduled trading day immediately preceding June 7, 2018.