Income taxes - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Income Taxes [Line Items]
Capital loss carryforwards	$ 388,900
Deferred tax assets valuation allowance	209,939	$ 184,413
Unrecognized tax benefits that, if recognized, would impact effective tax rate	10,800	12,500
Recognized interest and the release of penalty reserves	700	400	$ 3,400
Accrued interest and penalties payable related to unrecognized tax benefits	1,500	$ 1,700
Estimated decrease in gross unrecognized tax positions within the next 12 months, maximum	1,800
State
Income Taxes [Line Items]
Tax credit carryforward	$ 17,700
Tax years subject to examination	Tax years before 2013 remain subject to examination by certain states due to ongoing audits.
Federal
Income Taxes [Line Items]
Tax years subject to examination	The 2013 through 2016 tax years remain subject to examination by the Internal Revenue Service and state authorities.
Tax Year 2020
Income Taxes [Line Items]
Capital loss carryforwards subject to expiration	$ 361,500