Consolidated Income Statements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Operating revenues	$ 3,854.1	$ 4,059.4	$ 3,800.2
Cost of sales	1,399.0	1,660.7	1,436.4
Other operation and maintenance	1,384.9	1,356.4	1,417.3
Depreciation and amortization	304.0	278.3	230.6
Property and revenue taxes	117.3	113.6	110.0
Total operating expenses	3,205.2	3,409.0	3,194.3
Operating income	648.9	650.4	605.9
Equity in earnings of transmission affiliate	47.8	57.9	60.2
Other income, net	11.2	8.7	17.4
Interest expense	119.0	116.5	121.4
Other expense	(60.0)	(49.9)	(43.8)
Income before income taxes	588.9	600.5	562.1
Income tax expense	212.0	222.6	200.9
Net income	376.9	377.9	361.2
Preferred stock dividend requirements	1.2	1.2	1.2
Net income attributed to common shareholders	$ 375.7	$ 376.7	$ 360.0