Employee Benefit Plans (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) h shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)
Life insurance plan | Maximum | Retiree
Employee benefit plans
Benefits payable	$ 3,000
Medical plan | Maximum | Retiree
Employee benefit plans
Benefits payable	$ 15,000
Post retirement plan
Employee benefit plans
Minimum age required prior to amendment	55 years
Term of credited service before becoming eligible for retirement benefits before plan amendment	20 years
Net periodic post retirement benefit cost (recovery) recognized	$ (10,000)	$ (20,000)	$ (50,000)
Employee Stock Ownership and Profit Sharing Plan
Employee benefit plans
Minimum service period required to participate in Plan	90 days
Percentage of employee's deferred compensation up to which the employer matches (as a percent)	4.00%
Employer's matching contribution on the next 2% of employee contribution (as a percent)	50.00%
Employee contribution after matching contribution of employer (as a percent)	2.00%
Employee's eligible compensation on which employer pays fixed contribution (as a percent)	2.00%
Common stock held in relation to employee benefit plans (in shares) | shares	1,252,417	1,356,715
Minimum service period required to be eligible to get employer's contribution	5 years
Minimum working hours required per year to be eligible to get employer's contribution | h	1,000
Employee deferral that is always vested (as a percent)	100.00%
Contribution expense	$ 4,710,000	$ 4,570,000	$ 4,320,000