Income Taxes - Summary of Reconciliation of Income Tax Benefit with Amounts at the U.K. Statutory Income Tax Rate (Detail) - USD ($)		12 Months Ended
	Apr. 01, 2017	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory rate		$ (15,834,650)	$ (5,976,073)
Permanent differences-other		1,438,934	654,648
RTP and other adjustments		387,509	(152,948)
U.K. tax credit		1,707,489	539,136
U.S. tax credit		(436,250)	(363,665)
Foreign tax rate differential		(171,693)	(673,619)
State and local rate, net of federal tax		(1,159,522)	(446,683)
UK Rate Change (17% at expected DTA turn)		1,104,863	482,351
U.S. state rate change		(6,496)	993,998
Change in valuation allowance		12,495,426	4,942,855
Actual income tax benefit effective tax rate		$ (474,391)	$ 0
Statutory rate		19.00%	19.25%
Permanent differences-other		(1.73%)	(2.11%)
RTP and other adjustments		(0.46%)	0.49%
U.K. tax credit		(2.05%)	(1.74%)
U.S. tax credit		0.52%	1.17%
Foreign tax rate differential	19.00%	0.21%	2.17%
State and local rate, net of federal tax		1.39%	1.44%
UK Rate Change (17% at expected DTA turn)		(1.33%)	(1.55%)
U.S. state rate change		0.01%	(3.20%)
Change in valuation allowance		(14.99%)	(15.92%)
Actual income tax benefit effective tax rate		0.57%	0.00%