Pension and other postretirement benefits - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Oct. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase trend rate by 1%, increase the benefit obligation			$ 3,600
Increase trend rate by 1%, increase the benefit expenses			200
Decrease trend rate by 1%, decrease the benefit obligation			3,100
Decrease trend rate by 1%, decrease the benefit expenses			100
Defined benefit plans, annual average Medicare part D subsidy			200
Defined contribution plans, costs			$ 12,500	$ 12,000	$ 12,600
Equity Securities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage of securities			45.00%
Fixed Income Securities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage of securities			35.00%
Other Securities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage of securities			20.00%
United States Pension Plan of US Entity, Defined Benefit
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.76%		3.76%
Rate of return	8.00%	8.25%
United States Postretirement Benefit Plans of US Entity, Defined Benefit
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.60%		3.60%
Pension
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.60%		3.60%	4.20%
Rate of return			8.10%	8.10%	8.10%
Accumulated benefit obligation	$ 461,600		$ 461,600	$ 430,000
Expected employer contribution for year 2016			12,800
Accumulated other comprehensive income expected to be recognized over the next fiscal year for the Company's pension and postretirement benefit plans			$ 6,800
Postretirement Health Care Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.60%		3.60%	4.10%
Expected employer contribution for year 2016			$ 3,200
Accumulated other comprehensive income expected to be recognized over the next fiscal year for the Company's pension and postretirement benefit plans			$ 174	$ 284
Minimum
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Effective average duration to maintain			3 years
Maximum
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Effective average duration to maintain			10 years