Short-Term and Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Long-term Debt [Table Text Block]

Long-Term Debt
As of December 31	2017			2016
Millions
First Mortgage Bonds
1.83% Series Due 2018		$50.0			$50.0
8.17% Series Due 2019	42.0			42.0
5.28% Series Due 2020	35.0			35.0
2.80% Series Due 2020	40.0			40.0
4.85% Series Due 2021	15.0			15.0
3.02% Series Due 2021	60.0			60.0
3.40% Series Due 2022	75.0			75.0
6.02% Series Due 2023	75.0			75.0
3.69% Series Due 2024	60.0			60.0
4.90% Series Due 2025	30.0			30.0
5.10% Series Due 2025	30.0			30.0
3.20% Series Due 2026	75.0			75.0
5.99% Series Due 2027	60.0			60.0
3.30% Series Due 2028	40.0			40.0
3.74% Series Due 2029	50.0			50.0
3.86% Series Due 2030	60.0			60.0
5.69% Series Due 2036	50.0			50.0
6.00% Series Due 2040	35.0			35.0
5.82% Series Due 2040	45.0			45.0
4.08% Series Due 2042	85.0			85.0
4.21% Series Due 2043	60.0			60.0
4.95% Series Due 2044	40.0			40.0
5.05% Series Due 2044	40.0			40.0
4.39% Series Due 2044	50.0			50.0
Unsecured Term Loan Variable Rate Due 2017	—			125.0
Senior Unsecured Notes 5.99% Due 2017	—			50.0
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020	13.5			13.5
Unsecured Term Loan Variable Rate Due 2020	40.0			—
Armenia Mountain Senior Secured Notes 3.26% Due 2024	65.9			74.6
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025	27.8			27.8
Senior Unsecured Notes 3.11% Due 2027	80.0			—
SWL&P First Mortgage Bonds 4.15% Series Due 2028	15.0			15.0
Other Long-Term Debt, 3.11% – 5.37% Due 2018 – 2037	69.1			61.2
Unamortized Debt Issuance Costs	(10.0		)	(11.0		)
Total Long-Term Debt	1,503.3			1,558.1
Less: Due Within One Year	64.1			187.7
Net Long-Term Debt		$1,439.2			$1,370.4