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INCOME TAXES - Reconciliation of Income Tax Expense with Federal Income Taxes at the Statutory Rate (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Loss Carryforwards [Line Items]      
Federal statutory income tax rate 35.00% 35.00% 35.00%
Reconciliation of income tax expense with federal income taxes at the statutory rate      
Federal income tax $ 327,363 $ 356,670 $ 437,746
State income taxes, net of federal income tax benefit 19,850 25,993 29,507
Foreign rate difference 9,371 21,077 12,041
U.S. tax legislation impact (3,164) 0 0
Other - net (3,401) 11,150 (405)
Income tax expense $ 312,881 $ 386,220 $ 465,531
Effective tax rate 33.50% 37.90% 37.20%
Clean energy credit [Member]      
Reconciliation of income tax expense with federal income taxes at the statutory rate      
Clean energy credit $ (37,138) $ (28,670) $ (13,358)