Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,887	$ 2,615	$ 1,880
Interest cost	15,137	15,711	17,948
Expected return on plan assets	(26,809)	(24,786)	(25,940)
Net amortization and deferral	6,734	6,567	6,159
Net benefit expense (income)	(2,051)	107	47
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	279	355	495
Interest cost	1,577	1,595	1,967
Expected return on plan assets	0	0	0
Net amortization and deferral	275	454	216
Net benefit expense (income)	$ 2,131	$ 2,404	$ 2,678