Equity Incentive Program - RSUs (Details) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) $ / shares shares
Stock Options
Weighted Average Grant Date Fair Value (in dollars per share):
Unrecognized compensation expense \| $	$ 5.8
Weighted average period for compensation expense to be recognized	1 year 3 months 18 days
Restricted Stock Units (RSUs)
Number of Shares (in shares):
Beginning balance \| shares	2,202,576
Granted \| shares	1,707,911
Vested \| shares	(1,036,207)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period \| shares	(427,849)
Ending balance \| shares	2,446,431
Weighted Average Grant Date Fair Value (in dollars per share):
Beginning balance \| $ / shares	$ 16.54
Granted \| $ / shares	14.28
Vested \| $ / shares	16.95
Forfeited \| $ / shares	14.84
Ending balance \| $ / shares	$ 15.12
Unrecognized compensation expense \| $	$ 23.1
Weighted average period for compensation expense to be recognized	1 year 3 months 18 days
February 28, award 1 \| Share-based Compensation Award, Tranche Two [Member] \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percentage	50.00%
February 28, award 1 \| Share-based Compensation Award, Tranche Two [Member] \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percentage	50.00%
February 28, award 1 \| Third Anniversary of Grant Date \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percentage	50.00%
February 28, award 1 \| Third Anniversary of Grant Date \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percentage	50.00%

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 number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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