Income Taxes (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended				24 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016	Dec. 31, 2014 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2017 USD ($) item
Income tax examinations
Discrete income tax benefit	$ 1.3			$ 5.1
Effective tax rate
Effective income tax rate (as a percent)	(2507.15%)	(25.56%)		(1232.94%)	8.00%
Deferred tax assets and liabilities
Reduction in valuation allowance				$ 766.2
Net deferred tax assets	$ 635.9			635.9						$ 635.9
Net deferred income tax liability						$ 126.9
Valuation allowance	77.1			77.1						$ 77.1
Three year cumulative pretax income (loss)	76.6
Pretax income (loss)			$ (161.5)
Impairment charges			$ 155.3
Number of quarters with substantial pretax income in the past eight quarters | item										7
Goodwill and intangible asset impairment charges				29.9		$ 0.0	$ 40.0	$ 159.9	$ 798.3
Canadian Revenue Agency
Income tax examinations
Refund from tax audit settlement	$ 29.7			$ 29.7						$ 29.7