Retirement plans - Pension Costs (Detail) - Retirement Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	$ 872	$ 816	$ 920
Interest cost on benefit obligation	23,985	26,111	23,800
Expected return on plan assets	(26,322)	(26,764)	(31,464)
Amortization of prior service costs	635	670	673
Amortization of actuarial loss	8,357	7,615	6,335
Curtailment loss	26	0	0
Total pension expense for company-sponsored retirement plans	$ 7,553	$ 8,448	$ 264