Employee Benefit and Deferred Compensation Plans - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) point shares	Mar. 31, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum retirement age for benefits	55 years
Maximum retirement age for benefits	65 years
Eligible employee years of service	15 years
Number of points for eligibility | point	70
Minimum eligible age for medicare coverage	65 years
Life insurance coverage face value | $	$ 5,000
Age limit for life insurance coverage provided by the Company	70 years
Age at which retiree's pay for life insurance coverage at their sole expense	70 years
Option expiration period	10 years
Award vesting period	3 years
Stock options granted (in shares) | shares	0	0
Treasury shares reissued (in shares) | shares	71,747
Total stock-based compensation expense | $	$ 1,792,000	$ 1,174,000
Performance Based Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allowable variance	5.00%