Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current assets:
Other	$ 116.9	$ 125.7
Total current assets	1,043.1	1,011.2
Investments:
Other	58.4	57.5
Total investments	344.9	329.6
Other assets	1,759.5	1,445.1
Total assets	12,085.9	11,112.4	10,785.5
Current liabilities:
Current maturities of long-term debt	183.0	358.5
Commercial paper	141.3	279.4
Other	262.4	233.8
Total current liabilities	1,214.7	1,433.3
Long-term debt, net	3,606.7	2,977.8
Other liabilities:
Non-current deferred tax liabilities	2,321.1	2,112.7
Other	260.1	273.9
Total other liabilities	3,624.0	3,218.1
Common Equity:
Retained earnings	1,938.0	1,780.7
Shares in deferred compensation trust	(8.9)	(8.0)
Total common equity	3,438.7	3,281.4	3,134.9	3,013.0
Total liabilities and equity	12,085.9	11,112.4
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	95	72
Other	4.0	3.0
Total current assets	99.0	75.0
Investments:
Investments in consolidated subsidiaries	3,753	3,585
Other	15.0	14.0
Total investments	3,768.0	3,599.0
Other assets	10.0	6.0
Total assets	3,877.0	3,680.0
Current liabilities:
Current maturities of long-term debt	0	250.0
Commercial paper	141.0	96.0
Other	29.0	10.0
Total current liabilities	170.0	356.0
Long-term debt, net	250.0	0
Other liabilities:
Non-current deferred tax liabilities	11.0	39.0
Other	11.0	7.0
Total other liabilities	22.0	46.0
Common Equity:
Common stock and additional paid-in capital	1,510	1,509
Retained earnings	1,934.0	1,777.0
Shares in deferred compensation trust	(9.0)	(8.0)
Total common equity	3,435.0	3,278.0
Total liabilities and equity	$ 3,877.0	$ 3,680.0