Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ (44)	$ 129	$ (88)	$ 258
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,661	1,540	3,322	3,079
Expected return on plan assets	(2,361)	(2,321)	(4,722)	(4,642)
Amortization of net actuarial loss	652	936	1,304	1,873
Net periodic benefit cost	(48)	155	(96)	310
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	140	144	282	292
Expected return on plan assets	(198)	(226)	(399)	(457)
Amortization of net actuarial loss	62	56	125	113
Net periodic benefit cost	$ 4	$ (26)	$ 8	$ (52)