Summary of Significant Accounting Policies - ASC 606 Impact on Statement Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue
Revenue	$ 291.3	$ 297.5	$ 576.8	$ 597.5
Costs and expenses:
Selling, general and administrative	39.7	32.5	79.4	62.8
Total costs and expenses	243.4	241.2	743.7	482.1
Income (loss) from operations	47.9	56.3	(166.9)	115.4
Income (loss) before provision for income taxes	16.4	10.9	(227.5)	59.4
Net income (loss)	25.2	$ 5.0	(177.5)	$ 77.4
ASU 2014-09 | Adjustments
Revenue
Revenue	(0.1)		(2.0)
Costs and expenses:
Selling, general and administrative	(4.0)		(7.4)
Total costs and expenses	(4.0)		(7.4)
Income (loss) from operations	3.9		5.4
Income (loss) before provision for income taxes	3.9		5.4
Net income (loss)	3.9		5.4
ASU 2014-09 | Without adoption of Topic 606
Revenue
Revenue	291.4		578.8
Costs and expenses:
Selling, general and administrative	43.7		86.8
Total costs and expenses	247.4		751.1
Income (loss) from operations	44.0		(172.3)
Income (loss) before provision for income taxes	12.5		(232.9)
Net income (loss)	$ 21.3		$ (182.9)