Annual report pursuant to Section 13 and 15(d)

Selected Quarterly Data (Unaudited) (Details)

v3.8.0.1
Selected Quarterly Data (Unaudited) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly Financial Data [Abstract]                      
Revenues $ 194,795 $ 174,164 $ 193,860 $ 197,294 $ 214,901 $ 172,780 $ 160,473 $ 143,636 $ 760,113 $ 691,790 $ 533,982
Costs and operating expenses 181,908 161,927 178,855 183,098 201,615 159,157 148,594 130,895      
Operating income 12,887 12,237 15,005 14,196 13,286 13,623 11,879 12,741 54,325 51,529 50,539
Net income $ 17,357 $ 6,639 $ 7,807 $ 7,293 $ 7,184 $ 7,088 $ 5,969 $ 6,552 $ 39,096 $ 26,793 $ 24,918
Basic earnings per share:                      
Net income (in dollars per share) $ 1.60 $ 0.61 $ 0.72 $ 0.67 $ 0.67 $ 0.66 $ 0.55 $ 0.61 $ 3.61 $ 2.48 $ 2.32
Basic weighted average shares outstanding (in shares) 10,838,000 10,838,000 10,838,000 10,823,000 10,799,000 10,799,000 10,799,000 10,778,000 10,834,562 10,793,723 10,747,226
Diluted earnings per share:                      
Net income (in dollars per share) $ 1.59 $ 0.61 $ 0.72 $ 0.67 $ 0.66 $ 0.65 $ 0.55 $ 0.61 $ 3.60 $ 2.47 $ 2.31
Diluted weighted average shares outstanding (in shares) 10,903,000 10,857,000 10,862,000 10,849,000 10,853,000 10,826,000 10,826,000 10,806,000 10,867,834 10,828,152 10,787,270