Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,794	$ 1,819	$ 2,331
Interest cost	18,000	16,842	16,561
Expected return on plan assets	(25,006)	(23,122)	(20,245)
Net amortization and deferral	4,371	5,847	6,474
Curtailment gain	0	0	(197)
Settlement loss (gain)	0	0	106
Net benefit cost	(841)	1,386	5,030
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	424	663	704
Interest cost	2,169	2,707	2,122
Expected return on plan assets	0	0	0
Net amortization and deferral	(7)	1,348	761
Curtailment gain	0	0	0
Settlement loss (gain)	0	0	0
Net benefit cost	$ 2,586	$ 4,718	$ 3,587