Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Schedule of Estimated Amounts Amortized from Accumulated Other Comprehensive Income or Regulatory Assets into Net Periodic Benefit Cost (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	$ 22,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	1,000
PBOP
Defined Benefit Plan Disclosure [Line Items]
PBOP prior service cost	$ 1,300