Employee Benefit and Deferred Compensation Plans (Details Textual)	12 Months Ended
	Dec. 31, 2017 USD ($) point plan shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2007 officer
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Accumulated benefit obligation	$ 27,859,000	$ 28,012,000
Retiring age limit, minimum	55 years
Retiring age limit, maximum	65 years
Eligible employee from service (in years)	15 years
Number of points for eligibility (points) | point	70
Minimum eligible age for Medicare coverage	65 years
Life insurance coverage face value	$ 5,000
Life insurance maturity age	70 years
Additional life insurance maturity age	70 years
Expected company contributions	$ 214,000
Health care cost trend rate (percent)	5.50%
Employee deferrals (percent)	4.00%
Percentage of plan compensation as nondiscretionary contribution (percent)	5.00%
Percentage of plan compensation in excess of social security wage base (percent)	5.00%
Hours of service credited to the employees during the year	1000 hours
Employee deferrals amounts	$ 11,471,000	10,762,000	$ 9,271,000
Number of officers provided for | officer				4
Expiration period subsequent to retirement				15 years
Supplemental executive retirement liabilities totaled	$ 3,846,000	3,543,000
Number of deferred compensation plans, including deferred stock unit plan | plan	3
Number of conventional deferred compensation plans | plan	2
Company's deferred compensation plan expense	$ 1,935,000	1,537,000	1,225,000
Incentive compensation plan expense	$ 4,490,000	$ 2,307,000	$ 3,608,000
Option expiration period	10 years
Option exercisable period	3 years
Granted (shares) | shares	0	0	0
Intrinsic value of options exercised	$ 2,487,000	$ 8,323,000	$ 2,445,000
Fair value of options vested in period		78,000	235,000
Common shares reserved for issuance under employee benefit plans (shares) | shares	2,273,192
Stock compensation plan
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Compensation expense	$ 0	0	73,000
Unrecognized stock-based compensation expense	0
Performance- Based Restricted Stock
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Compensation expense	$ 5,293,000	3,117,000	3,884,000
Target performance level, allowable variance above and below target	5.00%
Time- Based Restricted Stock
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Unrecognized stock-based compensation expense	$ 2,105,000
Weighted average period over which unrecognized expense is expected to be recognized (in years)	1 year 7 months 6 days
Minimum | Other equity securities
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Equity income strategy (percent)	65.00%
Minimum | Debt Securities
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Equity income strategy (percent)	25.00%
Maximum | Other equity securities
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Equity income strategy (percent)	75.00%
Maximum | Debt Securities
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Equity income strategy (percent)	35.00%
Pension Benefits
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Company contribution to pension benefit plan	$ 0	0
Accumulated benefit obligation	27,859,000	28,012,000	$ 27,856,000
Other Postretirement Benefits Plan
Employee Benefit and Deferred Compensation Plans (Textual) [Abstract]
Company contribution to pension benefit plan	$ 119,000	$ 152,000