Shareholders' Equity - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020 £ / shares shares	Dec. 31, 2019 £ / shares shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Jun. 30, 2015 USD ($) shares
Class of Stock [Line Items]
Deferred shares, multiple of amount paid up on ordinary shares			100
Share buy-back program, authorized amount | $					$ 10,000,000
Share buy-back program, number of shares authorized (in shares)					2,700,000
Number of ordinary shares repurchased (in shares)	0	0
Number of treasury shares held (in shares)			350,335	352,499
Cost of treasury shares held | $			$ 4,000,000.0	$ 4,000,000.0
Ordinary shares
Class of Stock [Line Items]
Common stock, par value (in GBP per share) | £ / shares	£ 0.50	£ 0.50
Common stock, shares, outstanding, actively traded (in shares)			27,636,153	27,431,283
Employee stock ownership plan, shares held by trustees (in shares)			1,013,512	1,216,220
Deferred shares
Class of Stock [Line Items]
Common stock, par value (in GBP per share) | £ / shares	£ 0.0001	£ 0.0001