Investment in ATC - ATC Income Statement and Balance Sheet (Details) - ATC - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Income statement data
Revenues	$ 165.2	$ 160.0	$ 317.5	$ 323.3
Operating expenses	80.3	74.4	160.3	153.0
Other expense	24.3	21.9	48.6	43.5
Net income	60.6	$ 63.7	108.6	$ 126.8
Balance sheet data
Current assets	78.1		78.1		$ 66.4
Noncurrent assets	3,835.2		3,835.2		3,728.7
Total assets	3,913.3		3,913.3		3,795.1
Current liabilities	255.7		255.7		313.1
Long-term debt	1,800.0		1,800.0		1,701.0
Other noncurrent liabilities	197.7		197.7		163.8
Shareholders' equity	1,659.9		1,659.9		1,617.2
Total liabilities and shareholders' equity	$ 3,913.3		$ 3,913.3		$ 3,795.1