Postretirement Benefit Plans - Company's Asset Allocations for its U.K. Pension Plans by Asset Category (Detail) - United Kingdom [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total	$ 24,168	$ 20,336	$ 21,425
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	745	243
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	21,725	18,334
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	1,698	1,759
Cash [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	745	243
Cash [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	745	243
Pooled Pension Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	14,127	11,760
Pooled Pension Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	14,127	11,760
Pooled Fixed Pension Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	6,952	6,015
Pooled Fixed Pension Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	6,952	6,015
Real Estate Pooled Pension Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	646	559
Real Estate Pooled Pension Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	646	559
Insurance Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	1,698	1,759
Insurance Contracts [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	$ 1,698	$ 1,759