Financing Activities (Details) - USD ($) $ in Millions	6 Months Ended	9 Months Ended
	Jun. 30, 2016	Sep. 30, 2016
Debt Instrument [Line Items]
Debt Issued By Third Party		$ 144.7
Secured Debt [Member] | Secured Debt South Dakota Due June 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 60.0
Interest rate	2.80%
Maturity date	Jun. 15, 2026
Secured Debt [Member] | Secured Debt South Dakota Due 2018 [Member]
Debt Instrument [Line Items]
Interest rate	6.05%
Maturity date	May 01, 2018
Debt instrument, repurchased face amount	$ 55.0
Secured Debt [Member] | Secured Debt Montana Due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 144.7
Interest rate		2.00%
Maturity date		Aug. 01, 2023
Secured Debt [Member] | Repurchased Secured Debt Montana Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate		4.65%
Maturity date		Aug. 01, 2023
Debt instrument, repurchased face amount		$ 170.2
Secured Debt [Member] | Secured Debt South Dakota Due September 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 45.0
Interest rate		2.66%
Maturity date		Sep. 01, 2026