Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,615	$ 1,880	$ 1,794
Interest cost	15,711	17,948	18,000
Expected return on plan assets	(24,786)	(25,940)	(25,006)
Net amortization and deferral	6,567	6,159	4,371
Net benefit expense (income)	107	47	(841)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	355	495	424
Interest cost	1,595	1,967	2,169
Expected return on plan assets	0	0	0
Net amortization and deferral	454	216	(7)
Net benefit expense (income)	$ 2,404	$ 2,678	$ 2,586