Other Assets - Summary of Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Other Assets Noncurrent [Abstract]
Deferred financing costs on undrawn revolver, Gross Carrying Amount	$ 967	$ 967
Other regulatory assets Deferred financing costs, Gross Carrying Amount	28,570	28,570
Other regulatory assets Deferred costs recoverable in future years, Gross Carrying Amount	23,793	23,793
Other regulatory assets, Gross Carrying Amount	52,363	52,363
Investments, Gross Carrying Amount	2,519	2,519
Other assets, Gross Carrying Amount	55,849	55,849
Deferred financing costs on undrawn revolver, Accumulated Amortization	(639)	(591)
Other regulatory assets Deferred financing costs, Accumulated Amortization	(21,959)	(20,944)
Other regulatory assets, Accumulated Amortization	(21,959)	(20,944)
Other assets, Accumulated Amortization	(22,598)	(21,535)
Deferred financing costs on undrawn revolver, Net Carrying Amount	328	376
Other regulatory assets Deferred financing costs, Net Carrying Amount	6,611	7,626
Other regulatory assets Deferred costs recoverable in future years, Net Carrying Amount	23,793	23,793
Other regulatory assets, Net Carrying Amount	30,404	31,419
Investments, Net Carrying Amount	2,519	2,519
Other assets, Net Carrying Amount	$ 33,251	$ 34,314