Income Taxes - Reconciliation of Federal Statutory Income Tax Rate to Effective Tax Rate Additional Information (Detail)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory income tax rate	[1]	19.30%	20.00%	20.30%

[1] The statutory rate for 2017, 2016 and 2015 reflects the U.K. statutory rate of 19.3%, 20.0% and 20.3%, respectively.