Unfavorable Contracts Liability - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Unfavorable Contracts Liability [Abstract]
Percentage of retail rate charges to affiliates	60.00%
Unfavorable contract liability amortization period	5 years
Amortization into Wholesale revenues	$ 466	$ 25,200
Unfavorable contracts liability	12,585		$ 18,885
Amortization into Affiliate revenue share expense	$ 5,834