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, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (56,069)	$ (57,211)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(430,535)	(439,131)
Prior service cost	(8,040)	(8,373)
Less: amount recognized in regulatory assets	382,506	390,293
Recognized in AOCI	$ (56,069)	$ (57,211)