Number of share options (or share units) exercised during the current period.

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Name:	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Subsequent Events (Details) - GBP (£) £ in Thousands		1 Months Ended	3 Months Ended
Subsequent Events (Details) - GBP (£) £ in Thousands	Jul. 11, 2019	Aug. 06, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018
Common Stock
Subsequent Event [Line Items]
Exercise of stock options (in shares)			100,139	80,000	7,063	0
Subsequent Event
Subsequent Event [Line Items]
Rent expense, monthly	£ 20
Subsequent Event \| Common Stock
Subsequent Event [Line Items]
Exercise of stock options (in shares)		25,530