Derivatives (Open Commodity Derivative Contracts - Crude Oil) (Detail) - Crude Oil bbl in Thousands	12 Months Ended
	Dec. 31, 2016 bbl
Exchange Traded
Derivative [Line Items]
Percentage of derivative contracts expiring in the period	98.70%
Derivative contract expiration date	Mar. 31, 2017
Exchange Traded | Long
Derivative [Line Items]
Notional contracts (in thousands of total barrels)	53,028	[1]
Exchange Traded | Short
Derivative [Line Items]
Notional contracts (in thousands of total barrels)	52,373	[1]
Over the Counter | Short
Derivative [Line Items]
Notional contracts (in thousands of total barrels)	37

[1] of the exchange-traded contracts expire in the first quarter of 2017.