Risk Management and Trading Activities - Corporation's Unobservable Inputs Used in Fair Value Measurements (Detail)	Sep. 30, 2013	Dec. 31, 2012
Minimum [Member] | Contract prices [Member] | 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	71.23	79.35
Range of measurement of liabilities for recurring inputs	89.82	83.49
Minimum [Member] | Contract prices [Member] | Electricity [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	23.80	23.37
Range of measurement of liabilities for recurring inputs	24.75	25.01
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	(1.76)	(0.47)
Range of measurement of liabilities for recurring inputs	(1.75)	(0.72)
Minimum [Member] | Contract volatilities [Member] | Crude oil and refined petroleum products [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	14.00%	23.00%
Percentage range of measurement of liabilities for recurring inputs	14.00%	24.00%
Minimum [Member] | Contract volatilities [Member] | Electricity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	16.00%	18.00%
Minimum [Member] | Contract volatilities [Member] | Natural gas [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	19.00%	21.00%
Percentage range of measurement of liabilities for recurring inputs		21.00%
Maximum [Member] | Contract prices [Member] | 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	163.44	144.27
Range of measurement of liabilities for recurring inputs	130.53	133.38
Maximum [Member] | Contract prices [Member] | Electricity [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	102.25	79.27
Range of measurement of liabilities for recurring inputs	81.18	72.60
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	10.15	6.66
Range of measurement of liabilities for recurring inputs	10.15	6.66
Maximum [Member] | Contract volatilities [Member] | Crude oil and refined petroleum products [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	21.00%	27.00%
Percentage range of measurement of liabilities for recurring inputs	21.00%	27.00%
Maximum [Member] | Contract volatilities [Member] | Electricity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	29.00%	40.00%
Maximum [Member] | Contract volatilities [Member] | Natural gas [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	31.00%	36.00%
Percentage range of measurement of liabilities for recurring inputs		28.00%
Weighted Average [Member] | Contract prices [Member] | 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	113.54	113.06
Range of measurement of liabilities for recurring inputs	116.97	109.94
Weighted Average [Member] | Contract prices [Member] | Electricity [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range of measurement of assets for recurring inputs	37.09	40.81
Range of measurement of liabilities for recurring inputs	39.94	40.38
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	0.11	0.39
Range of measurement of liabilities for recurring inputs	0.73	1.26
Weighted Average [Member] | Contract volatilities [Member] | Crude oil and refined petroleum products [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	17.00%	26.00%
Percentage range of measurement of liabilities for recurring inputs	16.00%	26.00%
Weighted Average [Member] | Contract volatilities [Member] | Electricity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	23.00%	28.00%
Weighted Average [Member] | Contract volatilities [Member] | Natural gas [Member] | Commodity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage range of measurement of assets for recurring inputs	23.00%	25.00%
Percentage range of measurement of liabilities for recurring inputs		22.00%