Immaterial Restatement of Prior Period Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue	$ 126,111	$ 134,553
Operating expenses:
Income tax (benefit) expense	(2,991)	294
Total operating expenses		124,717
Net operating income	476	9,836
Net loss	$ (7,640)	$ (762)
Loss per share:
Basic (in dollars per share)	$ (0.16)	$ (0.02)
Diluted (in dollars per share)	$ (0.16)	$ (0.02)
Retained earnings	$ 374,920	$ 352,747	$ 392,053	$ 362,512
Total common stockholders’ equity	714,129	688,853	$ 730,157	699,221	$ 699,221
Operating activities:
Net loss	(7,640)	(762)
Other changes in noncurrent assets and liabilities	7,520	4,500
Net cash provided by operating activities	$ 20,141	28,252
Corrections
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue		2,306
Operating expenses:
Income tax (benefit) expense		523
Total operating expenses		523
Net operating income		1,783
Net loss		$ 1,783
Loss per share:
Basic (in dollars per share)		$ 0.03
Diluted (in dollars per share)		$ 0.03
Retained earnings		$ 7,542		5,759
Total common stockholders’ equity		7,542		5,759
Operating activities:
Net loss		1,783
Other changes in noncurrent assets and liabilities		(1,783)
Net cash provided by operating activities		0
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue		132,247
Operating expenses:
Income tax (benefit) expense		(229)
Total operating expenses		124,194
Net operating income		8,053
Net loss		$ (2,545)
Loss per share:
Basic (in dollars per share)		$ (0.05)
Diluted (in dollars per share)		$ (0.05)
Retained earnings		$ 345,205		356,753
Total common stockholders’ equity		681,311		$ 693,462
Operating activities:
Net loss		(2,545)
Other changes in noncurrent assets and liabilities		6,283
Net cash provided by operating activities		$ 28,252