Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in the Balance Sheet (Detail) (Swaps [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 4,298	$ 2,076
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	719	319
Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ (6)