Unconsolidated Affiliates - Condensed Financial Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement Data: (1)
Revenues	$ 590.1	$ 1,248.6	$ 1,225.7
Costs and expenses	477.1	1,192.4	1,132.7
Net income	113.0	56.2	$ 93.0
Balance Sheet Data: (1)
Current assets	156.2	570.2
Noncurrent assets	358.6	823.8
Current liabilities	98.7	423.0
Noncurrent liabilities	286.3	396.6
Partners’ and stockholders’ equity	$ 129.8	$ 574.4