Quarterly Results of Operations (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 141,083	$ 132,583	$ 135,381	$ 125,013	$ 123,270	$ 114,235	$ 107,358	$ 101,670	$ 534,060	$ 446,533	$ 429,099
Operating expenses	132,370	126,780	131,070	122,621	117,777	112,431	110,324	108,069
Operating (loss) income	8,713	5,803	4,311	2,392	5,493	1,804	(2,966)	(6,399)	21,219	(2,068)	(7,516)
Other, net	1,526	1,231	946	938	960	1,056	1,078	1,101
Income before income taxes	7,187	4,572	3,365	1,454	4,533	748	(4,044)	(7,500)	16,578	(6,263)	(11,218)
Income tax benefit	1,862	1,272	821	419	(10,291)	339	(1,198)	(2,610)	4,374	(13,760)	(3,519)
Net income (loss)	$ 5,325	$ 3,300	$ 2,544	$ 1,035	$ 14,824	$ 409	$ (2,846)	$ (4,890)	$ 12,204	$ 7,497	$ (7,699)
Denominator for basic earnings (loss) per share – weighted-average shares (in shares)	8,268	8,223	8,205	8,035	8,027	8,027	8,028	7,998	8,194	8,029	8,550
Basic earnings (loss) per share (in usd per share)	$ 0.65	$ 0.40	$ 0.31	$ 0.13	$ 1.85	$ 0.05	$ (0.35)	$ (0.61)	$ 1.49	$ 0.93	$ (0.90)
Denominator for diluted earnings (loss) per share – adjusted weighted-average shares and assumed conversions (in shares)	8,288	8,240	8,227	8,040	8,036	8,039	8,028	7,998	8,218	8,056	8,550
Diluted earnings (loss) per share (in usd per share)	$ 0.65	$ 0.40	$ 0.31	$ 0.13	$ 1.84	$ 0.05	$ (0.35)	$ (0.61)	$ 1.49	$ 0.93	$ (0.90)