Retirement plans - Pension Costs (Detail) - Retirement Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	$ 12	$ 872	$ 816
Interest cost on benefit obligation	21,337	23,985	26,111
Expected return on plan assets	(30,935)	(26,322)	(26,764)
Amortization of prior service costs	168	635	670
Amortization of actuarial loss	5,124	8,357	7,615
Pension payment timing related charges	7,498	26	0
Total pension expense for company-sponsored retirement plans	$ 3,204	$ 7,553	$ 8,448