INVESTMENT IN ATC INCOME STATEMENT AND BALANCE SHEET (Details) (ATC, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
ATC
Income statement data
Revenues	$ 152.1	$ 152.1	$ 303.9	$ 299.8
Operating expenses	69.9	71.7	139.7	141.3
Other expense	20.9	21.1	42.4	41.1
Net income	61.3	59.3	121.8	117.4
Balance sheet data
Current assets	77.1		77.1		63.1
Noncurrent assets	3,397.5		3,397.5		3,274.7
Total assets	3,474.6		3,474.6		3,337.8
Current liabilities	308.8		308.8		251.5
Long-term debt	1,550.0		1,550.0		1,550.0
Other noncurrent liabilities	129.7		129.7		95.8
Shareholders' equity	1,486.1		1,486.1		1,440.5
Total liabilities and shareholders' equity	$ 3,474.6		$ 3,474.6		$ 3,337.8