Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 01, 2016	Sep. 26, 2015	Oct. 01, 2016	Sep. 26, 2015	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Operating Loss Carryforwards Utilized During Period			$ 19.7
Operating Loss Carryforwards					$ 40.1
Taxable Income from Surrender of Policies			24.0
Deferred Tax Assets, Valuation Allowance	$ 12.6		$ 12.6		$ 19.2
Effective Income Tax Rate Reconciliation, Percent	(6.10%)	(1.80%)	(16.00%)	1.60%