SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Off System Sales (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2017
Eastern Missouri | First $2.0
Off System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	100.00%
Company Share	0.00%
Eastern Missouri | First $2.0 | Scenario, Forecast
Off System Sales [Line Items]
Customer Share		85.00%
Company Share		15.00%
Eastern Missouri | Next $2.0
Off System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	80.00%
Company Share	20.00%
Eastern Missouri | Next $2.0
Off System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	75.00%
Company Share	25.00%
Eastern Missouri | Amounts exceeding $6.0
Off System Sales [Line Items]
Pre-tax Income	$ 6.0
Customer Share	70.00%
Company Share	30.00%
Western Missouri | First $1.2
Off System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	85.00%
Company Share	15.00%
Western Missouri | Next $1.2
Off System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	80.00%
Company Share	20.00%
Western Missouri | Next $1.2
Off System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	75.00%
Company Share	25.00%
Western Missouri | Amounts exceeding $3.6
Off System Sales [Line Items]
Pre-tax Income	$ 3.6
Customer Share	70.00%
Company Share	30.00%