Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives (Detail) - Swaps - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other deferred credits and other long-term liabilities
Derivatives, Fair Value [Line Items]
Net Total	$ (24)
Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Net Total	4,697	$ 4,457
Deferred charges and other assets
Derivatives, Fair Value [Line Items]
Net Total	$ 131	$ 1,323