Defined Benefit Plans - Net Periodic Cost (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Compensation and Retirement Disclosure [Abstract]
Expected return on plan assets, net of expenses	$ (2,779)	$ (2,890)	$ (8,338)	$ (8,669)
Interest cost on projected benefit obligation	1,547	1,732	4,642	5,230
Net amortization and deferral	1,357	1,585	4,072	4,691
SERP settlement costs	0	0	0	187
Net periodic expense (benefit)	$ 125	$ 427	$ 376	$ 1,439