Summary Of Accumulated Other Comprehensive Income And Regulatory Assets (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension Plan Asset
Schedule of Pension and Other Postretirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss	$ 483,626	$ 409,129
Prior service cost (credit)	5,142	5,858
Net amount recognized	488,768	414,987
Regulatory assets	294,136	248,663
Accumulated other comprehensive income	194,632	166,324
Postretirement Benefit Plan Assets
Schedule of Pension and Other Postretirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss	183,087	164,305
Prior service cost (credit)	(16,708)	(16,864)
Net amount recognized	166,379	147,441
Regulatory assets	$ 166,379	$ 147,441