Postretirement Benefit Plans - Defined Benefit Pension Plans - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($)
Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance of trust assets	$ 1,677,000
United States [Member] | Unfunded Plan [Member] | Non-qualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions or payments to defined benefit plans	312,000
Employer contributions or payments	143,000
U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions or payments to defined benefit plans	476,000
Employer contributions or payments	$ 173,000