Income Taxes - Reconciliation (Details)			12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of the federal statutory rate to the effective income tax rate
Federal income tax rate	21.00%	35.00%	27.60%	34.00%
State income taxes			2.50%	2.10%
Share-based compensation			(0.70%)	(0.70%)
Permanent items			(0.10%)	(0.10%)
Research and development tax credits			0.00%	0.80%
Foreign taxes			(1.30%)	(1.00%)
Remeasurement of deferred taxes			(10.10%)
Effect of phased-in tax rate			(6.90%)
Uncertain tax positions			1.40%	(0.10%)
Valuation allowance and other			(12.00%)	(19.30%)
Effective rate			0.40%	15.70%