Equity, Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (59,163)	$ (61,520)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(431,747)	(448,555)
Prior service cost	(3,700)	(4,368)
Less: amount recognized in regulatory assets	376,284	391,403
Recognized in AOCI	$ (59,163)	$ (61,520)