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 Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2009
Jul. 31, 2011
Feb. 28, 2011
Dec. 31, 2008
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 $ 2.0 $ 1.9 $ 2.1        
Dart Helicopter Services LLC              
Schedule of Equity Method Investments [Line Items]              
Ownership 50.00%            
Additional investment           $ 5.0  
Dart Helicopter Services LLC | Equity Method Investee              
Schedule of Equity Method Investments [Line Items]              
Notes receivable to related party       $ 0.3      
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]              
Ownership         50.00%    
Dart Holding Company Ltd. | Equity Method Investee              
Schedule of Equity Method Investments [Line Items]              
Notes receivable to related party $ 2.8 $ 3.2     $ 5.1    
Notes receivable interest rate 4.00%