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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
Dec. 31, 2019
Dec. 31, 2018
Equity [Abstract]    
Net actuarial loss $ (483,074) $ (435,364)
Prior service cost (3,641) (3,033)
Less: amount recognized in regulatory assets 420,114 383,170
Recognized in AOCI $ (66,601) $ (55,227)