Share-Based Payments - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	1 Months Ended	3 Months Ended	12 Months Ended
Aug. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Share-Based Payments
Maximum number of shares authorized under plan (in shares)	500,000
ESPP Share Pool as a percentage of total number of ordinary shares outstanding			0.75%
Purchase discount as a percentage of current market price		15.00%

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