Postretirement Benefit Plans - Defined Benefit Pension Plans - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance of trust assets	$ 1,617,000	$ 1,617,000
United States [Member] | Funded Plan [Member] | Qualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	5,000,000	5,000,000
Expected contribution to defined benefit plans	0	0
United States [Member] | Unfunded Plan [Member] | Non-qualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		256,000
Expected contribution to defined benefit plans	312,000	312,000
U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		372,000
Expected contribution to defined benefit plans	$ 497,000	$ 497,000