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Unaudited Quarterly Financial Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Operating revenue $ 79,306 $ 112,344 $ 86,944 $ 61,112 $ 87,613 $ 82,955 $ 72,402 $ 62,112 $ 339,706 $ 305,082 $ 319,668
Operating Income (Loss) 16,389 36,346 29,800 10,581 31,685 18,260 17,210 12,805 93,116 79,960 92,878
Net income 13,727 18,959 16,775 3,378 16,192 9,534 7,461 4,695 52,839 37,882 51,806
Comprehensive income $ 13,834 $ 18,790 $ 15,520 $ 3,908 $ 16,368 $ 9,422 $ 7,206 $ 4,680 $ 52,052 $ 37,676 $ 50,936
Earnings per share                      
—Basic (in dollars per share) $ 0.67 $ 0.93 $ 0.82 $ 0.17 $ 0.79 $ 0.47 $ 0.37 $ 0.23 $ 2.59 $ 1.86 $ 2.56
—Diluted (in dollars per share) 0.67 0.92 0.82 0.16 0.79 0.46 0.36 0.23 $ 2.57 $ 1.85 $ 2.54
Dividends per share 0.20 0.20 0.20 0.20 0.19 0.20 0.19 0.20      
--- High (in dollars per share) [Member]                      
Earnings per share                      
Market Price, Stock 56.69 46.66 39.38 37.50 33.68 31.36 31.68 35.60      
--- Low (in dollars per share) [Member]                      
Earnings per share                      
Market Price, Stock $ 42.19 $ 38.13 $ 32.66 $ 29.35 $ 27.64 $ 27.73 $ 28.68 $ 30.09