Postretirement Benefit Plans - Defined Benefit Pension Plans - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($)
Defined Contribution Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance of trust assets	$ 2,035,000
United States [Member] | Unfunded Plan [Member] | Non-qualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions or payments to defined benefit plans	274,000
Employer contributions or payments	119,000
U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions or payments to defined benefit plans	495,000
Employer contributions or payments	$ 141,000