Share-Based Payments - Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Fair value of common stock (in dollars per share)	$ 25.25
Stock option and Stock Appreciation Rights
Number of Options and SAR
Outstanding at the beginning of the period (in shares)	2,699,007	2,080,749
Granted (in shares)	1,058,850	971,500
Exercised (in shares)	(85,694)	(205,640)
Forfeited (in share)	(28,075)	(147,602)
Outstanding at the end of the period (in shares)	3,644,088	2,699,007	2,080,749
Vested and expected to vest (in shares)	3,591,528
Exercisable (in shares)	1,503,004
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 8.94	$ 7.93
Granted (in dollars per share)	13.32	10.12
Exercised (in dollars per share)	6.51	4.56
Forfeited (in dollars per share)	12.23	8.60
Outstanding at the end of the period (in dollars per share)	10.25	$ 8.94	$ 7.93
Vested and expected to vest (in dollars per share)	10.22
Exercisable (in dollars per share)	$ 8.62
Stock Option
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Expected volatility, minimum (as a percent)	60.90%	60.90%	64.50%
Expected volatility, maximum (as a percent)	64.50%	64.60%	68.30%
Dividend Yield (as a percent)	0.00%	0.00%	0.00%
Expected term	6 years 3 months	6 years 3 months	6 years 3 months
Risk-free interest rate, minimum (as a percent)	1.14%	1.54%	1.67%
Risk-free interest rate, maximum (as a percent)	2.15%	1.74%	1.97%
Expected forfeiture rate (as a percent)	5.00%	5.00%	5.00%
Weighted-Average Remaining Contractual Term
Outstanding at the end of the period	7 years 7 months 2 days	7 years 11 months 1 day	8 years 15 days
Vested and expected to vest	7 years 6 months 26 days
Exercisable	6 years 5 months 27 days
Share-based payments, additional disclosure
Aggregate intrinsic value of options outstanding	$ 54.7	$ 12.6	$ 2.0
Aggregate intrinsic value of options vested and expected to vest	54.0	12.4	2.0
Aggregate intrinsic value of options exercisable	$ 25.0	$ 5.0	$ 1.5
Weighted-average, grant-date fair value of options granted (in dollars per share)	$ 7.66	$ 6.05	$ 5.79
Total fair value of the common stock vested	$ 3.9	$ 2.6	$ 1.9
Total intrinsic value of options exercised	1.1	1.6	$ 0.1
Total unrecognized compensation expense, net of estimated forfeitures	$ 9.8	$ 7.2
Weighted-average period over which total unrecognized compensation expense is expected to be recognized	2 years 8 months 12 days	2 years 6 months
Stock Option \| Minimum
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Fair value of common stock (in dollars per share)	$ 12.98	$ 9.13	$ 7.63
Stock Option \| Maximum
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Fair value of common stock (in dollars per share)	$ 22.80	$ 21.21	$ 10.02

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of fully vested and expected to vest options outstanding.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Price of a single share of a number of saleable stocks of a company.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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