Consolidated Statements Of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income
Balance, beginning of year, shares at Dec. 31, 2019		1,889
Balance, beginning of year at Dec. 31, 2019	$ 191,362	$ 0	$ 188,262	$ 3,100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	7,484		7,484
Dividends paid	(1,664)		(1,664)
Stock options and stock appreciation rights exercised (in shares)		3
Stock options and stock appreciation rights exercised	(1)		(1)
Share-based compensation expense related to stock appreciation rights	154		154
Accumulated postretirement benefit obligation adjustment	(32)			(32)
Net unrealized gain (loss) on investments	1,162			1,162
Balance, end of year, shares at Jun. 30, 2020		1,892
Balance, end of year at Jun. 30, 2020	$ 198,465	$ 0	194,235	4,230
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.88
Balance, beginning of year, shares at Mar. 31, 2020		1,891
Balance, beginning of year at Mar. 31, 2020	$ 183,647	$ 0	180,535	3,112
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,495		14,495
Dividends paid	(832)		(832)
Stock options and stock appreciation rights exercised (in shares)		1
Stock options and stock appreciation rights exercised	0		0
Share-based compensation expense related to stock appreciation rights	37		37
Net unrealized gain (loss) on investments	1,118			1,118
Balance, end of year, shares at Jun. 30, 2020		1,892
Balance, end of year at Jun. 30, 2020	$ 198,465	$ 0	194,235	4,230
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.44
Balance, beginning of year, shares at Dec. 31, 2020		1,892
Balance, beginning of year at Dec. 31, 2020	$ 200,422	$ 0	196,096	4,326
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	33,605		33,605
Dividends paid	(1,705)		(1,705)
Stock options and stock appreciation rights exercised (in shares)		2
Stock options and stock appreciation rights exercised	(1)		(1)
Share-based compensation expense related to stock appreciation rights	138		138
Accumulated postretirement benefit obligation adjustment	0
Net unrealized gain (loss) on investments	(521)			(521)
Balance, end of year, shares at Jun. 30, 2021		1,894
Balance, end of year at Jun. 30, 2021	$ 231,938	$ 0	228,133	3,805
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.90
Balance, beginning of year, shares at Mar. 31, 2021		1,894
Balance, beginning of year at Mar. 31, 2021	$ 212,873	$ 0	209,157	3,716
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,782		19,782
Dividends paid	(873)		(873)
Share-based compensation expense related to stock appreciation rights	67		67
Net unrealized gain (loss) on investments	89			89
Balance, end of year, shares at Jun. 30, 2021		1,894
Balance, end of year at Jun. 30, 2021	$ 231,938	$ 0	$ 228,133	$ 3,805
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.46