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, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (54,727)	$ (57,660)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(414,969)	(435,269)
Prior service cost	(6,037)	(7,038)
Less: amount recognized in regulatory assets	366,279	384,647
Recognized in AOCI	$ (54,727)	$ (57,660)