Investment in American Transmission Company - Summarized Financial Data (Details) - ATC - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Income statement data
Revenues	$ 154.3	$ 165.1	$ 318.5	$ 317.5
Operating expenses	81.7	80.3	160.8	160.3
Other expense	23.7	24.2	47.7	48.6
Net income	48.9	$ 60.6	110.0	$ 108.6
Balance sheet data
Current assets	83.9		83.9		$ 80.5
Noncurrent assets	4,104.6		4,104.6		3,948.3
Total assets	4,188.5		4,188.5		4,028.8
Current liabilities	382.5		382.5		330.3
Long-term debt	1,791.0		1,791.0		1,790.7
Other noncurrent liabilities	293.8		293.8		245.0
Shareholders' equity	1,721.2		1,721.2		1,662.8
Total liabilities and shareholders' equity	$ 4,188.5		$ 4,188.5		$ 4,028.8