Supplemental Cash Flow Information - Other Non-cash Investing and Financing Activities Affecting Recognized Assets and Liabilities but Not Result in Cash Receipts or Payments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Disclosure Of Non Cash Investing And Financing Information [Line Items]
(Decrease)/Increase to Utility Plant Accruals	$ 4,813	$ (2,741)	$ 8,514
Net Cost of Removal of Interim Retirements	35,983	31,626	4,592
Capital Lease Obligations	11,967	15,162	16,630
Asset Retirement Obligations	789	7,638	(1,872)
UED Secured Term Loan Prepayments			$ 3,188