Schedule II - Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Equity securities available-for-sale													$ 228,115
Equity Securities without Readily Determinable Fair Value, Amount												$ 1,200	0
Other long-term investments												19,316	13,648
Short-term investments												28,204	23,613
Total investments												1,546,992	1,540,392
Cash	$ 337			$ 347	$ 347			$ 307	$ 347	$ 307	$ 224	337	347	$ 307	$ 224
Accrued investment income												10,468	11,286
Accounts receivable												1,658	1,604
Income taxes recoverable												6,697	0
Amounts due from affiliate to settle inter-company transaction balances												5,154	0
Total assets												2,242,461	2,289,419
LIABILITIES
Amounts due affiliate to settle inter-company transaction balances												0	367
Deferred income taxes												4,908	15,020
Total liabilities												1,119,696	1,078,094
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 21,615,105 shares in 2018 and 21,455,545 shares in 2017												21,615	21,455
Additional paid-in capital												128,451	124,556
Accumulated other comprehensive income												1,620	83,384
Retained earnings												414,096	374,451
Total stockholders' equity												565,782	603,846	553,342	524,938
Total liabilities and stockholders' equity												1,685,478	1,681,940
REVENUES
Investment income									47,637	45,479	47,490
Net realized investment gains (losses) and, beginning in 2018, change in unrealized gains on equity investments									(41,252)	6,556	4,074
Total revenues	141,586	$ 189,125	$ 166,637	163,379	175,598	$ 167,196	$ 165,426	155,737	660,727	663,957	649,792
Other expenses									3,754	3,397	2,727
Income Tax Expense (Benefit)	(9,325)	5,296	(2,727)	(452)	(1,597)	(1,186)	1,725	1,636	(7,208)	578	17,004
Net income (loss)	(21,545)	19,148	(4,995)	(76)	26,184	746	5,504	6,804	(7,468)	39,238	46,203
Condensed Statements of Comprehensive Income [Abstract]
Net income (loss)	(21,545)	$ 19,148	$ (4,995)	(76)	26,184	$ 746	$ 5,504	6,804	(7,468)	39,238	46,203
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities not reflected in net income, net of deferred income taxes									(24,192)	27,278	(3,885)
Reclassification adjustment for net realized investment gains (losses) included in net income, net of income taxes									14,653	(8,326)	(6,736)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income taxes:
Net actuarial loss									333	958	1,549
Prior service credit									(2,489)	(2,047)	(2,399)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(2,156)	(1,089)	(850)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)									(4,885)	5,571	(542)
Prior service credit (cost)									1,050	96	(339)
Total change in funded status of affiliate's pension and postretirement benefit plans									(3,835)	5,667	(881)
Other comprehensive income (loss)									(15,530)	23,530	(12,352)
Total comprehensive income									(22,998)	62,768	33,851
Condensed Statements of Cash Flows [Abstract]
Net cash provided by operating activities									71,770	58,902	83,434
Cash flows from investing activities
Payments to Acquire Trading Securities Held-for-investment									(90,137)
Purchases of equity investments										(62,939)	(63,683)
Purchases of other long-term investments									(13,934)	(14,782)	(8,720)
Net (purchases) disposals of short-term investments									(4,591)	16,057	(1,071)
Net cash used in investing activities									(56,630)	(46,533)	(77,842)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									5,426	7,527	11,070
Repurchase of common stock									(1,455)	(1,858)	(383)
Dividends paid to stockholders (affiliated $(10,477), $(10,006) and $(9,182))									(19,121)	(17,998)	(16,196)
Net cash used in financing activities									(15,150)	(12,329)	(5,509)
Net increase (decrease) in cash									(10)	40	83
Cash at the beginning of the year				347				307	347	307	224
Cash at the end of the year	337				347				337	347	307
Income taxes recovered									(2,393)	(9,820)	(13,967)
Interest paid									654	337	337
Parent company [Member]
ASSETS
Common stock of subsidiaries (equity method)												556,977	599,036
Equity securities available-for-sale												0	2,000
Equity Securities without Readily Determinable Fair Value, Amount												1,200	0
Other long-term investments												3,811	0
Short-term investments												2,368	1,900
Total investments												564,356	602,936
Cash	206			255	255			184	255	184	136	206	255	$ 184	$ 136
Accrued investment income												3	1
Prepaid assets												119	136
Income taxes recoverable												510	777
Deferred income taxes												6	0
Amounts due from affiliate to settle inter-company transaction balances												705	0
Total assets												565,905	604,105
LIABILITIES
Accounts payable												123	80
Amounts due affiliate to settle inter-company transaction balances												0	160
Deferred income taxes												0	19
Total liabilities												123	259
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 21,615,105 shares in 2018 and 21,455,545 shares in 2017												21,615	21,455
Additional paid-in capital												128,451	124,556
Accumulated other comprehensive income												1,620	83,384
Retained earnings												414,096	374,451
Total stockholders' equity												565,782	603,846
Total liabilities and stockholders' equity												$ 565,905	$ 604,105
REVENUES
Dividends received from subsidiaries									21,124	5,719	9,707
Investment income									44	38	13
Net realized investment gains (losses) and, beginning in 2018, change in unrealized gains on equity investments									(90)	0	0
Total revenues									21,078	5,757	9,720
Other expenses									2,552	2,269	2,006
Income before income tax benefit and equity in undistributed net income (loss) of subsidiaries									18,526	3,488	7,714
Income Tax Expense (Benefit)									(535)	(794)	(698)
Income before equity in undistributed net income (loss) of subsidiaries									19,061	4,282	8,412
Equity in undistributed net income (loss) of subsidiaries									(26,529)	34,956	37,791
Net income (loss)									(7,468)	39,238	46,203
Condensed Statements of Comprehensive Income [Abstract]
Net income (loss)									(7,468)	39,238	46,203
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities not reflected in net income, net of deferred income taxes									(24,192)	27,278	(3,885)
Reclassification adjustment for net realized investment gains (losses) included in net income, net of income taxes									14,653	(8,326)	(6,736)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income taxes:
Net actuarial loss									333	958	1,549
Prior service credit									(2,489)	(2,047)	(2,399)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(2,156)	(1,089)	(850)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)									(4,885)	5,571	(542)
Prior service credit (cost)									1,050	96	(339)
Total change in funded status of affiliate's pension and postretirement benefit plans									(3,835)	5,667	(881)
Other comprehensive income (loss)									(15,530)	23,530	(12,352)
Total comprehensive income									(22,998)	62,768	33,851
Condensed Statements of Cash Flows [Abstract]
Net cash provided by operating activities									18,669	3,726	9,170
Cash flows from investing activities
Capital contributions to subsidiaries									0	(300)	(500)
Payments to Acquire Trading Securities Held-for-investment									(1,200)
Purchases of equity investments										0	(2,000)
Purchases of other long-term investments									(1,900)	0	0
Net (purchases) disposals of short-term investments									(468)	8,974	(1,113)
Net cash used in investing activities									(3,568)	8,674	(3,613)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									5,426	7,527	11,070
Repurchase of common stock									(1,455)	(1,858)	(383)
Dividends paid to stockholders (affiliated $(10,477), $(10,006) and $(9,182))									(19,121)	(17,998)	(16,196)
Net cash used in financing activities									(15,150)	(12,329)	(5,509)
Net increase (decrease) in cash									(49)	71	48
Cash at the beginning of the year				$ 255				$ 184	255	184	136
Cash at the end of the year	$ 206				$ 255				206	255	184
Income taxes recovered									777	700	716
Interest paid									$ 0	$ 0	$ 0