INVESTMENT IN ATC INCOME STATEMENT AND BALANCE SHEET (Details) (ATC, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
ATC
Income statement data
Revenues	$ 160.4	$ 150.3	$ 464.3	$ 450.1
Operating expenses	77.5	68.8	217.2	210.1
Other expense	20.2	21.0	62.6	62.1
Net income	62.7	60.5	184.5	177.9
Balance sheet data
Current assets	77.8		77.8		63.1
Noncurrent assets	3,455.4		3,455.4		3,274.7
Total assets	3,533.2		3,533.2		3,337.8
Current liabilities	353.6		353.6		251.5
Long-term debt	1,550.0		1,550.0		1,550.0
Other noncurrent liabilities	119.9		119.9		95.8
Shareholders' equity	1,509.7		1,509.7		1,440.5
Total liabilities and shareholders' equity	$ 3,533.2		$ 3,533.2		$ 3,337.8