Variable Interest Entities - Additional Information (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 MW	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $		$ 59	$ 46	$ 104
Power Purchase Agreements [Member] | Variable Interest Entity Not Primary Beneficiary [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $		$ 15	$ 16	$ 62
Minimum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range		121
Maximum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range		250
Maximum [Member] | Scenario Forecast [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range	360