Summary of Derivative Fair Value Balance (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Regulatory and other long-term assets	$ 4,855	$ 7,909
Long-term debt	5,035	7,715
Interest Rate Swap
Derivative [Line Items]
Long-term debt	5,035	7,715
Other
Derivative [Line Items]
Regulatory and other long-term assets	$ 4,855	$ 7,909