Fair Value of Financial Instruments (Schedule of Summarized Financial Information of Equity Method Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summarized Financial Information of Equity Method Investments
Current assets	$ 136.8	$ 89.5
Long-term assets	1,306.3	1,126.5
Total assets	1,443.1	1,216.0
Current liabilities	121.2	153.6
Long-term liabilities	976.5	986.0
Total liabilities	1,097.7	1,139.6
Revenue	114.5	0.0	$ 0.0
Net income (loss)	$ 64.5	$ (0.2)	$ (13.3)