Annual report pursuant to Section 13 and 15(d)

VARIABLE INTEREST ENTITIES AND EQUITY INVESTMENTS AND ADVANCES Dart Narrative (Details)

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VARIABLE INTEREST ENTITIES AND EQUITY INVESTMENTS AND ADVANCES Dart Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2009
Jul. 31, 2011
Feb. 28, 2011
Dec. 31, 2008
Schedule of Equity Method Investments [Line Items]              
Dividends received from equity investees   $ 0 $ 0        
Dart Helicopter Services LLC and Dart Holding Company Ltd. | Equity Method Investee | Products Purchased              
Schedule of Equity Method Investments [Line Items]              
Expenses from transactions with related party $ 2,000 2,000 $ 1,900        
Dart Helicopter Services LLC              
Schedule of Equity Method Investments [Line Items]              
Ownership 50.00%            
Additional investment           $ 5,000  
Dart Helicopter Services LLC | Equity Method Investee              
Schedule of Equity Method Investments [Line Items]              
Notes receivable to related party       $ 300      
Notes receivable interest rate       5.00%      
Dart Helicopter Services LLC | Era DHS LLC              
Schedule of Equity Method Investments [Line Items]              
Ownership             49.00%
Dart Holding Company Ltd.              
Schedule of Equity Method Investments [Line Items]              
Dividends received from equity investees $ 1,000            
Dart Holding Company Ltd. | Equity Method Investee              
Schedule of Equity Method Investments [Line Items]              
Notes receivable to related party $ 2,300 $ 2,800     $ 5,100    
Notes receivable interest rate 4.00%