Stock-Based Compensation (Details 2) (USD $)	12 Months Ended	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2015
Stock Option disclosures
Granted (in shares)	1,600,452	2,323,500	1,116,384
Number of Shares
Options outstanding (in shares)	3,632,996	1,817,839	891,751
Granted (in shares)	1,600,452	2,323,500	1,116,384
Exercised (in shares)	(283,057)	(371,743)	(30,250)
Forfeited or expired (in shares)	(550,285)	(136,600)	(160,046)
Options outstanding (in shares)	4,400,106	3,632,996	1,817,839
Vested and expected to vest (in shares)	3,891,511	3,402,306	1,692,915
Exercisable (in shares)	1,235,710	484,213	438,145
Weighted Average Exercise Price
Options outstanding (in dollars per share)	$ 7.94	$ 4.10	$ 5.15
Granted (in dollars per share)	$ 16.10	$ 10.53	$ 4.12
Exercised (in dollars per share)	$ 6.11	$ 4.37	$ 7.08
Forfeited or expired (in dollars per share)	$ 11.42	$ 10.49	$ 9.54
Options outstanding (in dollars per share)	$ 10.59	$ 7.94	$ 4.10
Vested and expected to vest (in dollars per share)	$ 10.32	$ 7.88	$ 4.11
Exercisable (in dollars per share)	$ 6.90	$ 4.25	$ 4.59
Weighted Average Remaining Contractual Life
Options outstanding	8 years 5 months 19 days
Vested and expected to vest	8 years 5 months 5 days
Exercisable	7 years 11 months 9 days
Aggregate Intrinsic Value
Options outstanding (in dollars)	$ 24,172,000
Vested and expected to vest (in dollars)	22,304,000
Exercisable (in dollars)	10,597,000
Stock options
Fair value and assumptions used in determining fair value of stock options
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected option term	6 years 3 months	6 years 3 months	6 years 3 months
Weighted-average fair value of stock options granted (in dollars per share)	$ 11.74	$ 8.16	$ 3.21
Stock options \| Minimum
Fair value and assumptions used in determining fair value of stock options
Volatility (as a percent)	83.00%	86.00%	99.00%
Risk-free interest rate (as a percent)	1.46%	0.65%	0.57%
Stock options \| Maximum
Fair value and assumptions used in determining fair value of stock options
Volatility (as a percent)	86.00%	96.00%	107.00%
Risk-free interest rate (as a percent)	1.83%	1.65%	0.99%
Performance shares
Number of Shares
Options outstanding (in shares)	396,667
Performance shares \| Forecast
Aggregate Intrinsic Value
Performance options expense				$ 1,500,000

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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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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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The estimated measure of the 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 risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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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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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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For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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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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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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