NOTES PAYABLE AND CREDIT AGREEMENTS - Alagasco's Short-term Borrowings (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 300,600,000	$ 82,300,000
Weighted average interest rate		0.70%	0.50%
Minimum borrowing outstanding		$ 180,100,000.0	$ 0
Maximum borrowing outstanding		488,500,000.0	300,500,000.0
Short-term debt	$ 287,100,000	$ 338,000,000	$ 287,100,000
Weighted average interest rate	0.50%	0.80%	0.50%
Line of Credit
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 65,600,000	$ 3,600,000
Weighted average interest rate		1.40%	1.40%
Minimum borrowing outstanding		$ 32,500,000.0	$ 0
Maximum borrowing outstanding		80,000,000.0	40,000,000
Short-term debt	$ 32,500,000	$ 74,000,000	$ 32,500,000
Weighted average interest rate	1.40%	1.50%	1.40%
Alagasco | Line of Credit
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 13,200,000	$ 22,300,000	$ 13,700,000
Weighted average interest rate	1.20%	1.10%	1.30%
Minimum borrowing outstanding	$ 9,000,000.0	$ 0	$ 0
Maximum borrowing outstanding	16,000,000.0	69,500,000.0	55,000,000.0
Short-term debt	$ 16,000,000	$ 31,000,000	$ 16,000,000
Weighted average interest rate	1.20%	1.20%	1.20%