Summary of Significant Accounting Policies (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Property, Plant, and Equipment [Line Items]
Outstanding checks in excess of bank account balances	388	$ 326
Furniture And Equipment [Member] | Minimum [Member]
Property, Plant, and Equipment [Line Items]
Useful life of assets	3 years
Furniture And Equipment [Member] | Maximum [Member]
Property, Plant, and Equipment [Line Items]
Useful life of assets	15 years
Buildings [Member] | Maximum [Member]
Property, Plant, and Equipment [Line Items]
Useful life of assets	40 years
Gathering Facilities [Member] | Maximum [Member]
Property, Plant, and Equipment [Line Items]
Useful life of assets	47 years