Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Defined Benefit Plans [Member]	Dec. 31, 2012 Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain			$ 74,853	$ 135,492		$ (398,028)	$ (423,662)
Prior service cost	(89)	(266)	2,157			(2,157)	(2,423)
Less: amount recognized in regulatory assets						350,864	373,615
Recognized in AOCI	$ (49,321)	$ (49,321)	$ (49,321)		$ (52,470)	$ (49,321)	$ (52,470)