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Unaudited Quarterly Financial Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Operating revenue $ 87,613 $ 82,955 $ 72,402 $ 62,112 $ 69,286 $ 125,430 $ 70,356 $ 54,596 $ 305,082 $ 319,668 $ 276,869
Operating Income (Loss) 31,685 18,260 17,210 12,805 15,589 59,114 12,121 6,054 79,960 92,878 53,407
Net income 16,192 9,534 7,461 4,695 5,688 38,366 6,846 906 37,882 51,806 22,384
Comprehensive income $ 16,368 $ 9,422 $ 7,206 $ 4,680 $ 6,021 $ 38,095 $ 5,716 $ 1,104 $ 37,676 $ 50,936 $ 23,461
Earnings per share                      
—Basic (in dollars per share) $ 0.79 $ 0.47 $ 0.37 $ 0.23 $ 0.28 $ 1.90 $ 0.34 $ 0.04 $ 1.86 $ 2.56 $ 1.13
—Diluted (in dollars per share) 0.79 0.46 0.36 0.23 0.28 1.88 0.34 0.04 $ 1.85 $ 2.54 $ 1.12
Dividends per share 0.19 0.20 0.19 0.20 0.19 0.19 0.19 0.19      
--- High (in dollars per share) [Member]                      
Earnings per share                      
Market Price, Stock 33.68 31.36 31.68 35.60 32.87 28.25 29.51 30.40      
--- Low (in dollars per share) [Member]                      
Earnings per share                      
Market Price, Stock $ 27.64 $ 27.73 $ 28.68 $ 30.09 $ 26.77 $ 25.64 $ 25.87 $ 27.84