LONG-TERM DEBT - Senior Revolving Credit Facility Due 2018 (Narrative) (Details) |
3 Months Ended | 12 Months Ended |
|---|---|---|
|
Sep. 30, 2013
USD ($)
|
Dec. 31, 2017
USD ($)
|
|
| Line of Credit Facility [Line Items] | ||
| Letters of credit which reduce availability under the facility | $ 95,900,000 | |
| Senior secured revolving credit facility | ||
| Line of Credit Facility [Line Items] | ||
| Term of senior secured revolving credit facility | 5 years | |
| Aggregate principal amount of revolving credit facility | $ 75,000,000.0 | |
| Amounts outstanding under revolving credit facility | 0 | |
| Letters of credit which reduce availability under the facility | $ 71,200,000 | |
| Secured leverage ratio | 1.5 | |
| Availability under facility as a percent of aggregate commitments | 75.00% |