Components of Net Periodic Benefit Cost for RIGP and Retiree Medical Plan (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Other changes in plan assets and benefit obligations recognized in OCI:
Total recognized in OCI	$ (2,843)	$ (7,019)
RIGP
Components of net periodic benefit cost:
Expected return on plan assets	(347)	(344)	(570)
Amortization of prior service cost benefit		(46)	(485)
Actuarial loss due to settlements	694		7,280
Amortization of unrecognized losses	1,121	967	1,069
Net periodic benefit costs	2,579	1,747	8,971
Service cost	284	263
Interest cost	827	906
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial loss (gain)	3,287	1,380	5,328
Amortization of net actuarial gain	(1,815)	(967)	(1,069)
Actuarial loss due to settlements			(7,280)
Amortization of prior service cost		46	485
Total recognized in OCI	1,472	459	(2,536)
Total recognized in net period benefit cost and OCI	4,051	2,206	6,435
RIGP | Net Periodic Benefit Costs
Components of net periodic benefit cost:
Service cost	284	263	495
Interest cost	827	907	1,182
Retiree Medical Plan
Components of net periodic benefit cost:
Recognized gain due to curtailments			(749)
Amortization of prior service cost benefit	(2,964)	(2,964)	(3,240)
Amortization of unrecognized losses	1,244	894	1,016
Net periodic benefit costs	510	206	(693)
Service cost	303	295
Interest cost	1,927	1,982
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial loss (gain)	(781)	4,490	875
Amortization of net actuarial gain	(1,244)	(894)	(1,016)
Amortization of prior service cost	2,964	2,964	3,240
Total recognized in OCI	939	6,560	3,099
Total recognized in net period benefit cost and OCI	1,449	6,766	2,406
Retiree Medical Plan | Net Periodic Benefit Costs
Components of net periodic benefit cost:
Service cost	303	294	339
Interest cost	$ 1,927	$ 1,982	$ 1,941