Estimated Benefit Payments from Expected Future Service (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
RIGP
Schedule of Pension and Other Postretirment Benefits Expected Benefit Payments [Line Items]
2012	$ 2,413
2013	1,575
2014	1,730
2015	2,070
2016	2,264
Thereafter	10,861
Retiree Medical Plan
Schedule of Pension and Other Postretirment Benefits Expected Benefit Payments [Line Items]
2012	3,141
2013	3,217
2014	3,229
2015	3,270
2016	3,236
Thereafter	$ 13,937