INVESTMENT IN ATC - INCOME STATEMENT AND BALANCE SHEET (Details) (ATC, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
ATC
Income statement data
Revenues	$ 160.0	$ 152.1	$ 323.3	$ 303.9
Operating expenses	74.4	69.9	153.0	139.7
Other expense	21.9	20.9	43.5	42.4
Net income	63.7	61.3	126.8	121.8
Balance sheet data
Current assets	80.6		80.6		80.7
Noncurrent assets	3,612.7		3,612.7		3,509.5
Total assets	3,693.3		3,693.3		3,590.2
Current liabilities	419.8		419.8		381.5
Long-term debt	1,550.0		1,550.0		1,550.0
Other noncurrent liabilities	135.3		135.3		126.1
Shareholders' equity	1,588.2		1,588.2		1,532.6
Total liabilities and shareholders' equity	$ 3,693.3		$ 3,693.3		$ 3,590.2