SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Concentration of Risk (Details) - USD ($) $ in Thousands	3 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Major customers
Operating revenues	$ 740,598	$ 564,087
Contract drilling \| Operating revenues \| Customer concentration \| International locations
Major customers
Concentration percentage	8.90%	11.20%
Contract drilling \| Operating revenues \| Customer concentration \| South America
Major customers
Concentration percentage	89.10%	96.20%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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