Assumptions Used in Determining Net Benefit Cost and Liabilities (Detail)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
RIGP
Weighted average expense assumptions:
Discount rate	4.70%	5.30%	5.50%
Expected return on plan assets	6.00%	6.00%	7.50%
Rate of compensation increase	4.00%	4.00%	4.00%
Weighted average balance sheet assumptions:
Discount rate	4.20%	4.70%
Expected return on plan assets	5.80%	6.00%
Retiree Medical Plan
Weighted average expense assumptions:
Discount rate	5.10%	5.80%	5.80%
Weighted average balance sheet assumptions:
Discount rate	4.60%	5.10%