Summary of Significant Accounting Policies - Life Insurance Policies (Details)	3 Months Ended				12 Months Ended
	Dec. 31, 2018 USD ($) Individual	Sep. 28, 2018 USD ($)	Mar. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Individual	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounting Policies [Abstract]
Number of former employees covered under insurance policies | Individual	18				18
Average age of former employees covered under insurance policies					75 years
Borrowing against cash surrender value of life insurance policies					$ 29,200,000
Gross cash surrender values of life insurance	$ 28,400,000				28,400,000
Loans on cash surrender value	25,800,000			$ 0	25,800,000	$ 0
Net cash surrender values of life insurance	2,626,000			31,547,000	2,626,000	31,547,000
Gross death benefit of life insurance contracts	37,600,000			42,200,000	37,600,000	42,200,000
Life insurance proceeds to be received upon the death of participants	11,600,000				11,600,000
Life insurance gain to be recognized upon death of participants	8,900,000				8,900,000
Non-taxable life insurance gain		$ 800,000		$ 400,000	807,000	$ 390,000
Life insurance proceeds	$ 1,000,000		$ 1,100,000		$ 2,151,000		$ 394,000