NOTES PAYABLE AND CREDIT AGREEMENTS - Alagasco's Short-term Borrowings (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 273,900,000	$ 300,600,000
Weighted average interest rate		0.90%	0.70%
Minimum borrowing outstanding		$ 73,100,000	$ 180,100,000
Maximum borrowing outstanding		427,200,000	488,500,000
Short-term debt		$ 398,700,000	$ 338,000,000
Weighted average interest rate		1.10%	0.80%
Line of Credit
Short-term Debt [Line Items]
Minimum borrowing outstanding		$ 0	$ 32,500,000
Maximum borrowing outstanding		82,000,000	80,000,000
Alagasco
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 42,600,000	$ 22,300,000
Weighted average interest rate		1.40%	1.10%
Minimum borrowing outstanding		$ 19,000,000	$ 0
Maximum borrowing outstanding		82,000,000	69,500,000
Short-term debt		$ 82,000,000	$ 31,000,000
Weighted average interest rate		1.50%	1.20%
Alagasco | Line of Credit
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 22,300,000	$ 30,200,000	$ 22,300,000
Weighted average interest rate	1.10%	1.40%	1.10%
Minimum borrowing outstanding		$ 0	$ 0
Maximum borrowing outstanding		82,000,000	69,500,000.0
Short-term debt		$ 82,000,000	$ 31,000,000
Weighted average interest rate		1.50%	1.20%
Alagasco | Notes Payable to Related Party
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 12,400,000	$ 0
Weighted average interest rate		1.40%	0.00%
Minimum borrowing outstanding		$ 0	$ 0
Maximum borrowing outstanding		61,900,000.0	0
Short-term debt		$ 0	$ 0
Weighted average interest rate		0.00%	0.00%