Financial Risk Management Activities - Fair Value Measurements - Additional Information (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total long-term debt, carrying value	$ 6,568
Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total long-term debt, fair value	$ 7,058