Investments (Summary Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Investments [Abstract]
Operating revenues	$ 152.1	$ 138.2	$ 299.8	$ 277.8
Operating income	80.4	75.2	158.5	151.7
Net income	$ 59.3	$ 55.7	$ 117.4	$ 109.9