Condensed Parent Company Financial Statements (Condensed Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Other	$ 101.7	$ 80.0
Total current assets	875.5	785.1
Investments:
Other	147.7	137.7
Total investments	467.8	431.3
Other assets	72.0	89.7
Total assets	16,700.7	15,426.0	$ 14,187.8
Current liabilities:
Commercial paper	337.4	441.2
Other	425.2	260.4
Total current liabilities	2,054.1	1,644.1
Other liabilities	298.9	287.2
Common Equity:
Retained earnings	2,765.4	2,545.9
Accumulated other comprehensive income	1.3	1.7
Shares in deferred compensation trust	(10.0)	(9.8)
Total common equity	5,205.1	4,585.7
Total liabilities and equity	16,700.7	15,426.0
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	27.0	23.0
Other	6.0	4.0
Total current assets	33.0	27.0
Investments:
Investments in consolidated subsidiaries	6,017.0	5,518.0
Other	1.0	1.0
Total investments	6,018.0	5,519.0
Other assets	89.0	81.0
Total assets	6,140.0	5,627.0
Current liabilities:
Commercial paper	169.0	285.0
Notes payable to affiliated companies	731.0	719.0
Other	19.0	21.0
Total current liabilities	919.0	1,025.0
Other liabilities	15.0	17.0
Common Equity:
Common stock and additional paid-in capital	2,448.0	2,048.0
Retained earnings	2,766.0	2,545.0
Accumulated other comprehensive income	2.0	2.0
Shares in deferred compensation trust	(10.0)	(10.0)
Total common equity	5,206.0	4,585.0
Total liabilities and equity	$ 6,140.0	$ 5,627.0