Financial Instruments and Credit Risk - Additional Information (Detail) - Customer Concentration Risk [Member] - Operating Revenues [Member]	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Royal Dutch Shell Plc [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percent of operating revenues contributed by major customers	49.00%	55.00%	67.00%
Freeport Mc Mo Ran Inc
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percent of operating revenues contributed by major customers	14.00%
Maximum percent of operating revenue accounted by other customers		10.00%	10.00%
Saudi Aramco
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percent of operating revenues contributed by major customers			10.00%
Maximum percent of operating revenue accounted by other customers	10.00%	10.00%