Summary of Significant Accounting Policies - Life Insurance Policies (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) Individual	Dec. 31, 2016 USD ($)
Accounting Policies [Abstract]
Number of former employees covered under insurance policies | Individual	20
Average age of former employees covered under insurance policies	73 years
Gross cash surrender values of life insurance	$ 30,700,000	$ 30,100,000
Loans on cash surrender value	0
Gross death benefit of life insurance contracts	41,900,000	$ 41,100,000
Life insurance proceeds to be received upon the death of participants	41,300,000
Life insurance gain to be recognized upon death of participants	$ 10,700,000