INVESTMENT IN ATC - INCOME STATEMENT AND BALANCE SHEET (Details) (ATC, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
ATC
Income statement data
Revenues	$ 163.7	$ 160.4	$ 487.0	$ 464.3
Operating expenses	76.6	77.5	229.6	217.2
Other expense	21.6	20.2	65.1	62.6
Net income	65.5	62.7	192.3	184.5
Balance sheet data
Current assets	72.6		72.6		80.7
Noncurrent assets	3,686.8		3,686.8		3,509.5
Total assets	3,759.4		3,759.4		3,590.2
Current liabilities	455.9		455.9		381.5
Long-term debt	1,550.0		1,550.0		1,550.0
Other noncurrent liabilities	140.5		140.5		126.1
Shareholders' equity	1,613.0		1,613.0		1,532.6
Total liabilities and shareholders' equity	$ 3,759.4		$ 3,759.4		$ 3,590.2