CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Accounts receivable and accrued unbilled revenues, reserves (in dollars)	$ 45.3	$ 43.5
Property, plant, and equipment, accumulated depreciation (in dollars)	$ 3,307.9	$ 3,114.7
Common stock, par value (in dollars per share)	$ 1
Common stock, shares authorized	200,000,000
Common stock, shares issued	79,757,715	78,287,906
Common stock, shares outstanding	79,289,748	77,902,467
Preferred stock of subsidiary, par value (in dollars per share)	$ 100
Preferred stock of subsidiary, shares authorized	1,000,000
Preferred stock of subsidiary, shares issued	511,882
Preferred stock of subsidiary, shares outstanding	510,495