Investment in ATC - ATC Summarized Financial Data (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Income statement data
Revenues	$ 164.5	$ 163.7	$ 482.0	$ 487.0
Operating expenses	78.0	76.6	238.3	229.6
Other expense	23.1	21.6	71.7	65.1
Net income	63.4	$ 65.5	172.0	$ 192.3
Balance sheet data
Current assets	80.8		80.8		$ 66.4
Noncurrent assets	3,900.9		3,900.9		3,728.7
Total assets	3,981.7		3,981.7		3,795.1
Current liabilities	294.8		294.8		313.1
Long-term debt	1,800.0		1,800.0		1,701.0
Other noncurrent liabilities	207.1		207.1		163.8
Shareholders' equity	1,679.8		1,679.8		1,617.2
Total liabilities and shareholders' equity	$ 3,981.7		$ 3,981.7		$ 3,795.1