Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013	[2]
Current assets:
Other		$ 95.5	$ 79.7
Total current assets		877.1	826.8
Investments:
Other		20.0	53.0
Total investments		337.6	346.3
Other assets		22.6	14.6
Total assets		13,373.8	12,495.2		$ 12,063.5
Current liabilities:
Current maturities of long-term debt		4.6	313.4
Commercial paper		244.1	159.8
Other		222.3	243.4
Total current liabilities		1,162.0	1,359.3
Other liabilities		279.3	306.0
Common Equity:
Retained earnings		2,177.0	2,068.9
Shares in deferred compensation trust	[1]	(10.0)	(8.5)
Total common equity		3,862.0	3,724.1	[2]	$ 3,438.7	[2]	$ 3,281.4
Total liabilities and equity		13,373.8	12,495.2
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies		74.0	93.0
Other		5.0	9.0
Total current assets		79.0	102.0
Investments:
Investments in consolidated subsidiaries		4,211.0	3,999.0
Other		2.0	14.0
Total investments		4,213.0	4,013.0
Other assets		64.0	20.0
Total assets		4,356.0	4,135.0
Current liabilities:
Current maturities of long-term debt		0.0	250.0
Commercial paper		192.0	140.0
Notes payable to affiliated companies		275.0	0.0
Other		12.0	12.0
Total current liabilities		479.0	402.0
Other liabilities		18.0	12.0
Common Equity:
Common stock and additional paid-in capital		1,695.0	1,664.0
Retained earnings		2,174.0	2,066.0
Shares in deferred compensation trust		(10.0)	(9.0)
Total common equity		3,859.0	3,721.0
Total liabilities and equity		$ 4,356.0	$ 4,135.0

[1] Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.

[2] Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.