Income Taxes Income Taxes - Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected tax at statutory federal tax rates	$ 201.4	$ 205.7	$ 209.8
State income taxes net of federal tax benefit	31.8	31.0	33.0
Production tax credits	(16.5)	(17.8)	(17.4)
Domestic production activities deduction	(7.8)	(7.8)	0.0
AFUDC - Equity	(1.5)	(2.0)	(1.5)
Investment tax credit restored	(1.1)	(1.1)	(1.1)
Other, net	4.7	4.0	(0.2)
Total income tax expense	$ 211.0	$ 212.0	$ 222.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected tax at statutory federal tax rates	35.00%	35.00%	35.00%
State income taxes net of federal tax benefit	5.50%	5.30%	5.50%
Production tax credits	(2.80%)	(3.00%)	(2.90%)
Domestic production activities deduction	(1.40%)	(1.30%)	(0.00%)
AFUDC - Equity	(0.30%)	(0.30%)	(0.20%)
Investment tax credit restored	(0.20%)	(0.20%)	(0.20%)
Other, net	0.80%	0.50%	(0.10%)
Total income tax expense	36.60%	36.00%	37.10%