Summary of Significant Accounting Policies - Life Insurance Policies (Details)	3 Months Ended
	Mar. 30, 2018 USD ($) Individual	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Supplementary Information For Corporate Owned Life Insurance [Line Items]
Number of former employees covered under insurance policies | Individual	19
Average age of former employees covered under insurance policies	74 years
Net cash surrender values of life insurance	$ 24,890,000		$ 31,547,000
Loans on cash surrender value	6,100,000		0
Gross cash surrender values of life insurance	31,000,000
Gross death benefit of life insurance contracts	41,100,000		$ 42,200,000
Life insurance proceeds to be received upon the death of participants	34,400,000
Life insurance gain to be recognized upon death of participants	$ 9,500,000
Subsequent Event [Member]
Supplementary Information For Corporate Owned Life Insurance [Line Items]
Borrowing against cash surrender value of life insurance policies		$ 22,400,000