CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
CONDENSED CONSOLIDATED BALANCE SHEETS
Accounts receivable and accrued unbilled revenues, reserves (in dollars)	$ 47.2	$ 43.5
Property, plant, and equipment, accumulated depreciation (in dollars)	$ 3,225.5	$ 3,114.7
Common stock, par value (in dollars per share)	$ 1
Common stock, shares authorized	200,000,000
Common stock, shares issued	78,809,948	78,287,906
Common stock, shares outstanding	78,353,096	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