Reconciliation of Projected Benefit Obligations, Plan Assets and the Funded Status of the RIGP and the Retiree Medical Plan (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
RIGP
Change in benefit obligation:
Benefit obligation at beginning of year	$ 17,959	$ 19,103
Service cost	284	263
Interest cost	827	906
Actuarial loss (gain)	3,689	1,281
Settlements	(1,375)		(14,000)
Benefit payments	(93)	(3,594)
Benefit obligation at end of year	21,291	17,959	19,103
Change in plan assets:
Beginning balance, January 1	4,807	5,427
Actual return on plan assets	890	244
Employer contribution	2,389	2,730
Settlements	(1,375)
Benefits paid	(93)	(3,594)
Ending balance, December 31	6,618	4,807	5,427
Funded status at end of year	(14,673)	(13,152)
Retiree Medical Plan
Change in benefit obligation:
Benefit obligation at beginning of year	39,835	35,449
Service cost	303	295
Interest cost	1,927	1,982
Plan participants' contributions	486	397
Actuarial loss (gain)	(781)	4,490
Benefit payments	(2,773)	(2,778)
Benefit obligation at end of year	38,997	39,835
Change in plan assets:
Plan participants' contributions	486	397
Employer contribution	2,287	2,381
Benefits paid	(2,773)	(2,778)
Funded status at end of year	$ (38,997)	$ (39,835)