Income Taxes Statutory Rate 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, amount	$ 526.4	$ 375.5	$ 332.5
State income taxes net of federal tax benefit, amount	72.8	73.1	50.5
Production tax credits, amount	(15.7)	(17.4)	(17.4)
AFUDC - Equity, amount	(8.8)	(7.1)	(1.9)
Investment tax credit restored, amount	(4.9)	(1.7)	(1.1)
Other, net, Amount	3.3	(11.4)	0.9
Total income tax expense, amount	$ 566.5	$ 433.8	$ 361.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected tax at statutory federal tax rates, effective tax rate	35.00%	35.00%	35.00%
State income taxes net of federal tax benefit, effective tax rate	4.80%	6.80%	5.30%
Production tax credits, effective tax rate	(1.10%)	(1.60%)	(1.80%)
AFUDC - Equity, effective tax rate	(0.60%)	(0.70%)	(0.20%)
Investment tax credit restored, Effective Tax Rate	(0.30%)	(0.20%)	(0.20%)
Other, net, Effective Tax Rate	(0.20%)	1.10%	(0.10%)
Total Income Tax Expense, Effective Tax Rate	37.60%	40.40%	38.00%