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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	$ 944,346	$ 848,341
United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	383,831	326,833
Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	18,819	332,169
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	332,385	282,237
Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	925,527	516,172
Significant Other Observable Inputs Level (2) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	13,376	10,800
Significant Unobservable Inputs Level (3)
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	38,070	33,796	$ 27,062	$ 25,812
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	38,070	33,796
Cash and cash equivalents | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	18,819	8,491
Cash and cash equivalents | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	1,578	1,078
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	18,819	8,491
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	1,578	1,078
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
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
Equity securities | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		184,690
Equity securities | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	69,547	60,185
Equity securities | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		184,690
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	69,547	60,185
Equity securities | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		0
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 States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		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
Fixed income securities | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		138,988
Fixed income securities | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	261,260	220,974
Fixed income securities | Quoted Prices in Active Markets for Identical Assets Level (1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		138,988
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	261,260	220,974
Fixed income securities | Significant Other Observable Inputs Level (2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		0
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 States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets		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
Collective Trust Funds - Equity | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	381,696	347,249
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	381,696	347,249
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 - Fixed income | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	490,955	168,923
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	490,955	168,923
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 - Real Estate | United States
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	52,876
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
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	52,876
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
Other investments | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	51,446	44,596
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	13,376	10,800
Other investments | Significant Unobservable Inputs Level (3) | United Kingdom plan
Defined Benefit Plan Disclosure [Line Items]
Fair market value of plan assets	$ 38,070	$ 33,796