Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ 53,785	$ 57,211
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(413,345)	(439,131)
Prior service cost	(7,372)	(8,373)
Less: amount recognized in regulatory assets	366,932	390,293
Recognized in AOCI	$ (53,785)	$ (57,211)