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QUARTERLY FINANCIAL 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
Schedule of Quarterly Financial Information [Line Items]                      
Revenues $ 462,394 $ 247,142 $ 279,689 $ 550,408 $ 440,754 $ 232,191 $ 245,923 $ 508,364 $ 1,539,633 $ 1,427,232 $ 1,547,692
Net margin                 925,132 885,435 841,733
Operating income 89,512 40,780 44,052 125,132 78,534 30,892 43,621 116,073 299,476 269,120 239,129
Net income $ 47,119 $ 18,797 $ 20,623 $ 76,456 $ 42,315 $ 12,737 $ 20,300 $ 64,743 $ 162,995 $ 140,095 $ 119,030
Basic $ 0.90 $ 0.36 $ 0.39 $ 1.45 $ 0.81 $ 0.24 $ 0.39 $ 1.23 $ 3.10 $ 2.67 $ 2.26
Diluted $ 0.89 $ 0.36 $ 0.39 $ 1.44 $ 0.80 $ 0.24 $ 0.38 $ 1.22 $ 3.08 $ 2.65 $ 2.24