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Immaterial Restatement of Prior Period Financial Statements (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]                  
Operating revenue $ 179,031   $ 174,938   $ 305,142 $ 309,491      
Operating expenses:                  
Income taxes 4,321   4,870   1,330 5,164      
Total operating expenses     148,099     272,816      
Net operating income 27,013   26,839   27,489 36,675      
Net income $ 16,996 $ (7,640) $ 14,805 $ (762) $ 9,356 $ 14,043      
Earnings per share:                  
Basic (in dollars per share) $ 0.35   $ 0.31   $ 0.19 $ 0.29      
Diluted (in dollars per share) $ 0.35   $ 0.31   $ 0.19 $ 0.29      
Retained earnings $ 382,409   $ 358,538 352,747 $ 382,409 $ 358,538 $ 392,053 $ 362,512  
Total common stockholders’ equity 723,164 714,129 695,257 688,853 723,164 695,257 $ 730,157 699,221 $ 699,221
Operating activities:                  
Net income (loss) $ 16,996 $ (7,640) 14,805 (762) 9,356 14,043      
Other changes in noncurrent assets and liabilities         11,494 10,139      
Net cash provided by operating activities         $ 43,391 49,571      
Corrections                  
Error Corrections and Prior Period Adjustments Restatement [Line Items]                  
Operating revenue     2,306     4,612      
Operating expenses:                  
Income taxes     523     1,046      
Total operating expenses     523     1,046      
Net operating income     1,783     3,566      
Net income     $ 1,783     $ 3,566      
Earnings per share:                  
Basic (in dollars per share)     $ 0.04     $ 0.07      
Diluted (in dollars per share)     $ 0.04     $ 0.07      
Retained earnings     $ 9,325 7,542   $ 9,325   5,759  
Total common stockholders’ equity     9,325 7,542   9,325   5,759  
Operating activities:                  
Net income (loss)     1,783     3,566      
Other changes in noncurrent assets and liabilities           (3,566)      
Net cash provided by operating activities           0      
As Previously Reported                  
Error Corrections and Prior Period Adjustments Restatement [Line Items]                  
Operating revenue     172,632     304,879      
Operating expenses:                  
Income taxes     4,347     4,118      
Total operating expenses     147,576     271,770      
Net operating income     25,056     33,109      
Net income     $ 13,022     $ 10,477      
Earnings per share:                  
Basic (in dollars per share)     $ 0.27     $ 0.22      
Diluted (in dollars per share)     $ 0.27     $ 0.22      
Retained earnings     $ 349,213 345,205   $ 349,213   356,753  
Total common stockholders’ equity     685,932 $ 681,311   685,932   $ 693,462  
Operating activities:                  
Net income (loss)     $ 13,022     10,477      
Other changes in noncurrent assets and liabilities           13,705      
Net cash provided by operating activities           $ 49,571