Nuclear Plant (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Investments in decommissioning trust fund [Line Items]
Cost Basis	$ 158.5		$ 158.5		$ 152.8
Unrealized Gains	61.0		61.0		50.5
Unrealized Losses	(1.2)		(1.2)		(2.6)
Fair Value	218.3		$ 218.3		200.7	$ 199.0
Weighted average maturity of debt securities (in years)			8 years
Realized gains	0.6	$ 0.6	$ 1.5	$ 3.2
Realized losses	(0.3)	$ (0.5)	(1.3)	$ (2.3)
Equity Securities [Member]
Investments in decommissioning trust fund [Line Items]
Cost Basis	92.4		92.4		89.6
Unrealized Gains	56.2		56.2		47.9
Unrealized Losses	(1.2)		(1.2)		(2.1)
Fair Value	147.4		147.4		135.4
Debt Securities [Member]
Investments in decommissioning trust fund [Line Items]
Cost Basis	64.5		64.5		59.6
Unrealized Gains	4.8		4.8		2.6
Unrealized Losses	0.0		0.0		(0.5)
Fair Value	69.3		69.3		61.7
Other Securities [Member]
Investments in decommissioning trust fund [Line Items]
Cost Basis	1.6		1.6		3.6
Fair Value	$ 1.6		$ 1.6		$ 3.6