Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Other	$ 80.0	$ 46.4
Total current assets	785.1	905.1
Investments:
Other	137.7	121.9
Total investments	431.3	396.1
Other assets	89.7	69.7
Total assets	15,426.0	14,187.8	$ 13,373.8
Current liabilities:
Commercial paper	441.2	320.2
Other	260.4	260.8
Total current liabilities	1,644.1	2,149.0
Other liabilities	287.2	306.4
Common Equity:
Retained earnings	2,545.9	2,346.0
Accumulated other comprehensive income (loss)	1.7	(0.5)
Shares in deferred compensation trust	(9.8)	(11.1)
Total common equity	4,585.7	4,182.2	$ 4,062.0
Total liabilities and equity	15,426.0	14,187.8
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	23.0	50.0
Other	4.0	7.0
Total current assets	27.0	57.0
Investments:
Investments in consolidated subsidiaries	5,518.0	4,676.0
Other	1.0	2.0
Total investments	5,519.0	4,678.0
Other assets	81.0	78.0
Total assets	5,627.0	4,813.0
Current liabilities:
Commercial paper	285.0	295.0
Notes payable to affiliated companies	719.0	305.0
Other	21.0	12.0
Total current liabilities	1,025.0	612.0
Other liabilities	17.0	20.0
Common Equity:
Common stock and additional paid-in capital	2,048.0	1,848.0
Retained earnings	2,545.0	2,344.0
Accumulated other comprehensive income (loss)	2.0	0.0
Shares in deferred compensation trust	(10.0)	(11.0)
Total common equity	4,585.0	4,181.0
Total liabilities and equity	$ 5,627.0	$ 4,813.0