Pension and other postretirement benefits - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 328,830
Fair value of plan assets, end of year	315,951	$ 328,830
Pension
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	447,964	367,731
Service cost	1,880	1,794	$ 1,819
Interest cost	17,948	18,000	16,842
Actuarial loss (gain)	(22,880)	82,922
Currency translation	(2,721)	(2,973)
Benefits paid	(18,682)	(17,988)
Medicare Part D reimbursement	0	0
Administrative costs	(1,773)	(1,522)
Projected benefit obligation, end of year	421,736	447,964	367,731
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	328,830	305,481
Actual return on plan assets	(4,460)	34,332
Contributions	12,797	9,539
Benefits paid	(18,682)	(17,988)
Settlements paid	0	0
Currency translation	(761)	(1,012)
Fair value of plan assets, end of year	315,951	328,830	305,481
Funded status, end of year	(105,785)	(119,134)
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	53,154	52,448
Service cost	495	424	663
Interest cost	1,967	2,169	2,707
Actuarial loss (gain)	(3,914)	1,273
Currency translation	0	0
Benefits paid	(3,216)	(3,287)
Medicare Part D reimbursement	130	127
Administrative costs	0	0
Projected benefit obligation, end of year	48,616	53,154	$ 52,448
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Benefits paid	(3,216)	(3,287)
Funded status, end of year	$ (48,616)	$ (53,154)