Annual report pursuant to Section 13 and 15(d)

NATURE OF OPERATIONS AND ACCOUNTING POLICIES ERA Savings Plan (Details)

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NATURE OF OPERATIONS AND ACCOUNTING POLICIES ERA Savings Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Percent of employer contribution 3.00%    
Employer matching contribution, percent 100.00%    
Employer matching contribution, percent of employees' gross pay 3.00%    
Employer contribution $ 2.4 $ 2.8 $ 3.2