Credit Arrangements - Summary of Credit Facilities (Detail)		12 Months Ended
	Dec. 15, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)
Revolving credit facility [Member]
Line Of Credit Facility [Line Items]
Facility		$ 1,000,000,000
Interest Rate Description		LIBOR in the relevant currency borrowed plus a margin (margin of 2.00% at December 31, 2017)
Revolving credit facility [Member] | LIBOR [Member]
Line Of Credit Facility [Line Items]
Rate		2.00%	2.00%
Receivables Financing Facility [Member]
Line Of Credit Facility [Line Items]
Facility		$ 25,000,000
Interest Rate Description		LIBOR Market Index Rate (1.56% at December 31, 2017) plus 0.90%
Rate		1.56%	1.56%
Receivables Financing Facility [Member] | LIBOR [Member]
Line Of Credit Facility [Line Items]
Interest Rate Description		In addition, the applicable margin (over LIBOR) changed to 90 bps regardless of the Company’s credit rating. Prior to the amendment, the margin was based on the Company’s credit rating and could range from 85 bps to 135 bps.
Interest Rate spread on base rate	0.90%	0.90%
General Banking Facility [Member]
Line Of Credit Facility [Line Items]
Facility		$ 13,000,000	£ 10,000,000
Interest Rate Description		Bank’s base rate (0.50% at December 31, 2017) plus 1%
General Banking Facility [Member] | Base Rate [Member]
Line Of Credit Facility [Line Items]
Rate		0.50%	0.50%
Interest Rate spread on base rate		1.00%