Employee Benefit and Deferred Compensation Plans - (Details Textual)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) shares	Jun. 30, 2016 USD ($) shares	Jun. 30, 2017 USD ($) point shares	Jun. 30, 2016 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Retiring age limit, minimum			55 years
Retiring age limit, maximum			65 years
Eligible employee years of service (in years)			15 years
Number of points for eligibility | point			70
Minimum eligible age for medicare coverage			65 years
Life insurance coverage face value | $			$ 5,000
Life insurance coverage age (until)			70 years
Life insurance coverage at retiree expense after age			70 years
Option expiration period (in years)			10 years
Award vesting period (in years)			3 years
Stock options granted (shares) | shares	0	0	0	0
Treasury shares reissued (shares) | shares			92,610
Total stock-based compensation expense | $	$ 1,237,000	$ 856,000	$ 2,411,000	$ 1,715,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period (in years)			1 year