Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets:
Net operating loss carryforwards	$ 20,646,185	$ 9,462,690
Other	1,414,690	539,008
Tax Credit	1,133,656	697,406
Deferred tax assets	23,194,531	10,699,105
Less: valuation allowance	(23,194,531)	(10,699,105)
Net deferred tax asset	0	0
U.K. [Member]
Deferred Tax Assets:
Net operating loss carryforwards	16,000,329	6,909,754
Other	(46,604)	152,554
Deferred tax assets	15,953,725	7,062,308
Less: valuation allowance	(15,953,725)	(7,062,308)
Net deferred tax asset	0	0
Federal [Member]
Deferred Tax Assets:
Net operating loss carryforwards	4,645,856	2,552,937
Other	1,461,294	386,454
Tax Credit	1,133,656	697,406
Deferred tax assets	7,240,806	3,636,797
Less: valuation allowance	(7,240,806)	(3,636,797)
Net deferred tax asset	$ 0	$ 0