Common Stock, Common Stock Warrants and Stock Option Plan - Restricted Stock Units - Activity (Details) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding, beginning of period (in shares)	1,451,507	1,154,980	509,894
Granted (in shares)	958,825	978,851	925,578
Released (in shares)	(143,625)	(512,703)	(264,568)
Forfeited/expired (in shares)	(13,526)	(169,621)	(15,924)
Outstanding, end of period (in shares)	2,253,181	1,451,507	1,154,980	509,894
Weighted Average Grant Date Fair Value
Outstanding, beginning of period (in dollars per share)	$ 6.18	$ 5.50	$ 2.84
Granted (in dollars per share)	7.88	6.35	6.55
Released (in dollars per share)	5.40	5.31	4.02
Forfeited/expired (in dollars per share)	7.67	5.15	8.34
Outstanding, end of period (in dollars per share)	$ 6.94	$ 6.18	$ 5.50	$ 2.84
Weighted Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years)	1 year 6 months	1 year 3 months 18 days	1 year 4 months 24 days	1 year 4 months 24 days
Aggregate Intrinsic Value
Outstanding, end of period	$ 25,182	$ 12,314	$ 5,082	$ 3,288

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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 weighted average fair value at grant date for nonvested equity-based awards issued 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 non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Weighted average remaining contractual term for equity-based awards excluding options, 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 vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Intrinsic value of outstanding award under share-based payment arrangement. Excludes share and unit options.

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