Supplemental Consolidated Financial Statement Information - Disaggregated Revenues (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Number of business segments | segment									1
Operating revenues	$ 33,557,000	$ 36,976,000	$ 24,076,000	$ 51,164,000	$ 27,670,000	$ 40,448,000	$ 36,158,000	$ 49,880,000	$ 145,773,000	$ 154,156,000	$ 156,532,000
United States
Operating revenues									129,452,000	137,101,000	134,442,000
Canada
Operating revenues									$ 16,321,000	$ 17,055,000	$ 22,090,000