Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,880	$ 1,794	$ 1,819
Interest cost	17,948	18,000	16,842
Expected return on plan assets	(25,940)	(25,006)	(23,122)
Net amortization and deferral	6,159	4,371	5,847
Net benefit expense (income)	47	(841)	1,386
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	495	424	663
Interest cost	1,967	2,169	2,707
Expected return on plan assets	0	0	0
Net amortization and deferral	216	(7)	1,348
Net benefit expense (income)	$ 2,678	$ 2,586	$ 4,718