INVESTMENT IN ATC (Details 2) (ATC, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
ATC
Income statement data
Revenues	$ 151.8	$ 147.7
Operating expenses	69.8	69.6
Other expense	21.5	20.0
Net income	60.5	58.1
Balance sheet data
Current assets	69.3		63.1
Noncurrent assets	3,325.3		3,274.7
Total assets	3,394.6		3,337.8
Current liabilities	261.5		251.5
Long-term debt	1,550.0		1,550.0
Other noncurrent liabilities	124.8		95.8
Shareholders' equity	1,458.3		1,440.5
Total liabilities and shareholders' equity	$ 3,394.6		$ 3,337.8