Earnings Per Common Share and Share Awards (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Common Share And Share Awards [Line Items]
Incremental dilutive potential common shares, calculated using treasury stock method (in shares)	9,517	7,382	5,439
Anti-dilutive shares excluded from computation of diluted earnings per share	4,500	0	0
Intrinsic values of options exercised	$ 473,000	$ 117,000	$ 104,000
Options and SARs vested	4,500
Fair value of options and SARs vested	$ 219,315
Weighted-average fair values for SARs issued (dollars per share)	$ 55.40	$ 28.75	$ 31.16
Compensation expense relating to SARs or options vesting	$ 219,000	$ 132,000	$ 138,000
Total unrecognized compensation cost related to unvested share-based compensation arrangements granted under stock award plans	$ 62,000
Weighted-average period of unrecognized compensation cost recognition	3 months
Number of stock options or SARs granted where exercise price was less than market price on date of grant	0
Maximum
Earnings Per Common Share And Share Awards [Line Items]
Maximum shares of Company stock to be granted to key employees or directors	250,000