Pensions and Postretirement Benefits Other than Pensions - Assets of U.S and U.K Plans Using Fair Value Hierarchy (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	$ 1,117,650	$ 1,047,214
United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	407,336	377,536
Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	41,621	23,867
Quoted Prices in Active Markets for Identical Assets Level (1) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	363,786	333,497
Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	30,019	22,410
Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	14,737	13,859
Significant Unobservable Inputs Level (3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	28,813	30,180	$ 28,938	$ 38,070
Fair Value Measured at Net Asset Value Per Share | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	1,046,010	1,000,937
Fair Value Measured at Net Asset Value Per Share | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	41,621	23,867
Cash and cash equivalents | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	1,755	1,802
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	41,621	23,867
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets Level (1) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	1,755	1,802
Cash and cash equivalents | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | Significant Unobservable Inputs Level (3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | Fair Value Measured at Net Asset Value Per Share | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Cash and cash equivalents | Fair Value Measured at Net Asset Value Per Share | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Equity | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	319,143	309,794
Collective Trust Funds - Equity | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Equity | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Equity | Significant Unobservable Inputs Level (3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Equity | Fair Value Measured at Net Asset Value Per Share | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	319,143	309,794
Collective Trust Funds - Fixed income | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	718,702	682,279
Collective Trust Funds - Fixed income | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Fixed income | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	30,019	22,410
Collective Trust Funds - Fixed income | Significant Unobservable Inputs Level (3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Fixed income | Fair Value Measured at Net Asset Value Per Share | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	688,683	659,869
Collective Trust Funds - Real Estate | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	38,184	31,274
Collective Trust Funds - Real Estate | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Real Estate | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Real Estate | Significant Unobservable Inputs Level (3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Collective Trust Funds - Real Estate | Fair Value Measured at Net Asset Value Per Share | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	38,184	31,274
Equity securities | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	73,963	72,503
Equity securities | Quoted Prices in Active Markets for Identical Assets Level (1) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	73,963	72,503
Equity securities | Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Equity securities | Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Equity securities | Fair Value Measured at Net Asset Value Per Share | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Fixed income securities | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	288,068	259,192
Fixed income securities | Quoted Prices in Active Markets for Identical Assets Level (1) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	288,068	259,192
Fixed income securities | Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Fixed income securities | Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Fixed income securities | Fair Value Measured at Net Asset Value Per Share | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Other investments | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	43,550	44,039
Other investments | Quoted Prices in Active Markets for Identical Assets Level (1) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	0	0
Other investments | Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	14,737	13,859
Other investments | Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	28,813	30,180
Other investments | Fair Value Measured at Net Asset Value Per Share | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	$ 0	$ 0