Organization and Basis of Presentation - Additional Information (Detail)	1 Months Ended
	Sep. 30, 2015 $ / shares	Dec. 31, 2017 $ / shares	Dec. 31, 2016 $ / shares	Sep. 30, 2015 £ / shares
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Common stock, par value | $ / shares
Preferred shares no par value | $ / shares
Share split ratio	5.913
Preferred stock, par value | £ / shares				£ 0.01
Common stock, par value | £ / shares				£ 0.01