Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Other	$ 79.7	$ 116.9
Total current assets	826.8	1,043.1
Investments:
Other	53.0	58.4
Total investments	346.3	344.9
Other assets	1,803.0	1,737.1
Total assets	12,495.2	12,063.5	$ 11,092.5
Current liabilities:
Current maturities of long-term debt	313.4	183.0
Commercial paper	159.8	141.3
Other	296.6	262.4
Total current liabilities	1,359.3	1,214.7
Long-term debt, net (excluding current portion)	3,522.2	3,584.3
Other liabilities	306.0	260.1
Common Equity:
Retained earnings	2,068.9	1,938.0
Shares in deferred compensation trust	(8.5)	(8.9)
Total common equity	3,724.1	3,438.7	$ 3,281.4	$ 3,134.9
Total liabilities and equity	12,495.2	12,063.5
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	93.0	95.0
Other	9.0	25.0
Total current assets	102.0	120.0
Investments:
Investments in consolidated subsidiaries	3,999.0	3,721.0
Other	14.0	15.0
Total investments	4,013.0	3,736.0
Other assets	20.0	10.0
Total assets	4,135.0	3,866.0
Current liabilities:
Current maturities of long-term debt	250.0	0.0
Commercial paper	140.0	141.0
Other	12.0	29.0
Total current liabilities	402.0	170.0
Long-term debt, net (excluding current portion)	0.0	250.0
Other liabilities	12.0	11.0
Common Equity:
Common stock and additional paid-in capital	1,664.0	1,510.0
Retained earnings	2,066.0	1,934.0
Shares in deferred compensation trust	(9.0)	(9.0)
Total common equity	3,721.0	3,435.0
Total liabilities and equity	$ 4,135.0	$ 3,866.0