Pensions and Other Post-retirement Benefits - Summary of Pension Plan Assets Measured at Fair Value on Recurring Basis by Fair Value Hierarchy (Detail) - Pension Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	$ 492,677	$ 433,262	$ 419,088
Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	492,677	433,262
Plan assets at NAV	132,828	87,481
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	255,665	267,961
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	86,350	62,872
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	17,834	14,948
Fair Value, Measurements, Recurring | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	278,606	304,460
Plan assets at NAV	64,840	62,094
Fair Value, Measurements, Recurring | Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	213,766	242,161
Fair Value, Measurements, Recurring | Equity securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	205
Fair Value, Measurements, Recurring | Equity securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	127,128	87,776
Fair Value, Measurements, Recurring | Fixed income securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	40,778	25,109
Fair Value, Measurements, Recurring | Fixed income securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	86,350	62,667
Fair Value, Measurements, Recurring | Fixed income securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Pooled investment funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	60,014	20,156
Plan assets at NAV	60,014	20,156
Fair Value, Measurements, Recurring | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	17,834	14,948
Fair Value, Measurements, Recurring | Insurance contracts | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Insurance contracts | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Insurance contracts | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	17,834	14,948
Fair Value, Measurements, Recurring | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	9,095	5,922
Plan assets at NAV	7,974	5,231
Fair Value, Measurements, Recurring | Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	1,121	691
Fair Value, Measurements, Recurring | Cash and cash equivalents | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Cash and cash equivalents | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value	$ 0	$ 0