Financial Risk Management and Trading Activities - Commodity Contracts with a Fair Value (Detail)	Sep. 30, 2014	Dec. 31, 2013
Minimum [Member] | Contract Prices | Crude Oil and Refined Petroleum Products [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	73.58	78.45
Range of measurement of liabilities for recurring inputs	82.54	57.45
Minimum [Member] | Contract Prices | Electricity (Millions of megawatt hours)[Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs		19.52
Range of measurement of liabilities for recurring inputs		26.48
Minimum [Member] | Basis Prices [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	(0.65)	(4.99)
Range of measurement of liabilities for recurring inputs		(1.90)
Minimum [Member] | Contract volatilities [Member] | Crude Oil and Refined Petroleum Products [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	16.00%	16.00%
Percentage range of measurement of liabilities for recurring inputs	16.00%	16.00%
Minimum [Member] | Contract volatilities [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	18.00%	17.00%
Percentage range of measurement of liabilities for recurring inputs		34.00%
Minimum [Member] | Contract volatilities [Member] | Electricity (Millions of megawatt hours)[Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs		16.00%
Percentage range of measurement of liabilities for recurring inputs		16.00%
Maximum [Member] | Contract Prices | Crude Oil and Refined Petroleum Products [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	119.76	228.86
Range of measurement of liabilities for recurring inputs	122.95	183.89
Maximum [Member] | Contract Prices | Electricity (Millions of megawatt hours)[Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs		165.75
Range of measurement of liabilities for recurring inputs		155.33
Maximum [Member] | Basis Prices [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	4.01	18.10
Range of measurement of liabilities for recurring inputs		18.00
Maximum [Member] | Contract volatilities [Member] | Crude Oil and Refined Petroleum Products [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	19.00%	18.00%
Percentage range of measurement of liabilities for recurring inputs	20.00%	17.00%
Maximum [Member] | Contract volatilities [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	39.00%	35.00%
Percentage range of measurement of liabilities for recurring inputs		35.00%
Maximum [Member] | Contract volatilities [Member] | Electricity (Millions of megawatt hours)[Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs		36.00%
Percentage range of measurement of liabilities for recurring inputs		36.00%
Weighted Average [Member] | Contract Prices | Crude Oil and Refined Petroleum Products [Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	95.97	118.68
Range of measurement of liabilities for recurring inputs	102.77	122.54
Weighted Average [Member] | Contract Prices | Electricity (Millions of megawatt hours)[Member] | Commodity [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs		45.76
Range of measurement of liabilities for recurring inputs		43.12
Weighted Average [Member] | Basis Prices [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	3.57	0.23
Range of measurement of liabilities for recurring inputs		(0.62)
Weighted Average [Member] | Contract volatilities [Member] | Crude Oil and Refined Petroleum Products [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	17.00%	17.00%
Percentage range of measurement of liabilities for recurring inputs	19.00%	17.00%
Weighted Average [Member] | Contract volatilities [Member] | Natural Gas [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	29.00%	22.00%
Percentage range of measurement of liabilities for recurring inputs		35.00%
Weighted Average [Member] | Contract volatilities [Member] | Electricity (Millions of megawatt hours)[Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs		23.00%
Percentage range of measurement of liabilities for recurring inputs		22.00%