Gains and Losses Reclassified from AOCI to Income and Change in Value Recognized in Other Comprehensive Income (OCI) on Derivatives (Detail) (Derivatives Designated as Hedging Instruments, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in OCI on Derivatives	$ (104,763)	$ (13,856)
Interest Rate Swap | Interest and Debt Expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from AOCI to Income	(920)	(964)
Futures Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in OCI on Derivatives	(46)	(929)
Futures Contracts | Cost of product sales and natural gas storage services
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from AOCI to Income	$ (250)	$ (428)