Summary of Significant Accounting Policies - Other (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies
Emerging growth company, annual revenue threshold	$ 1,070.0
Value of common stock that is held by non-affiliates, that must be in excess of, in determining whether the Company is deemed to be a large accelerated filer and whether the Company will remain an emerging growth company	700.0
Value of non-convertible debt securities, in excess of, that was issued during the prior three-year period, in determining whether the Company will remain an emerging growth company	$ 1,000.0
Period for issuance of non-convertible debt securities in determining the threshold value of securities issued and whether the Company will remain an emerging growth company	3 years
Accounts receivable
Allowance for doubtful accounts	$ 1.3	$ 1.1
Bad debt expense	0.3	0.6	$ 0.7
Goodwill
Goodwill impairment	$ 0.0	$ 0.0	$ 1.6