Schedule II - Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Equity securities available-for-sale, at fair value (cost $2,000 and $0)												$ 213,839	$ 206,243
Short-term investments												39,670	38,599
Total investments												1,465,714	1,415,797
Cash	$ 307			$ 224	$ 224			$ 383	$ 224	$ 383	$ 239	307	224	$ 383	$ 239
Accrued investment income												11,050	10,789
Accounts receivable												2,076	804
Income taxes recoverable												0	1,735
Total assets												2,131,694	2,055,437
LIABILITIES
Amounts due affiliate to settle inter-company transaction balances												11,222	6,408
Deferred income taxes												11,321	19,029
Total liabilities												1,035,471	1,011,017
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 21,222,535 shares in 2016 and 20,780,439 shares in 2015												21,223	20,781
Additional paid-in capital												119,054	108,747
Accumulated other comprehensive income												46,081	58,433	81,662
Retained earnings												366,984	336,977
Total stockholders' equity												553,342	524,938	502,886	455,210
Total liabilities and stockholders' equity												1,588,813	1,535,955
REVENUES
Investment income (loss)									47,490	45,582	46,465
Total revenues	168,398	$ 162,378	$ 160,336	153,871	147,479	$ 165,104	$ 158,808	152,335	644,983	623,726	594,467
Other expenses									2,727	2,690	2,377
Income Tax Expense (Benefit)	7,904	918	1,959	6,223	4,028	4,732	3,127	9,607	17,004	21,494	10,915
Net income	21,292	4,129	6,128	14,654	9,895	11,189	8,748	20,330	46,203	50,162	29,992
Condensed Statements of Comprehensive Income [Abstract]
Net income	21,292	$ 4,129	$ 6,128	14,654	9,895	$ 11,189	$ 8,748	20,330	46,203	50,162	29,992
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income taxes									(3,885)	(13,037)	34,663
Reclassification adjustment for realized investment gains included in net income, net of income taxes									(6,736)	(4,956)	(4,677)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income taxes:
Net actuarial loss									1,549	863	375
Prior service credit									(2,399)	(2,150)	(2,149)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(850)	(1,287)	(1,774)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial loss									(542)	(3,637)	(5,525)
Prior service cost									(339)	(312)	(35)
Total change in funded status of affiliate's pension and postretirement benefit plans									(881)	(3,949)	(5,560)
Other comprehensive income (loss)									(12,352)	(23,229)	22,652
Total comprehensive income									33,851	26,933	52,644
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities									83,434	85,612	91,815
Cash flows from investing activities
Purchases of equity securities available-for-sale									(63,683)	(83,098)	(50,154)
Net (purchases) disposals of short-term investments									(1,071)	14,663	2,904
Net cash used in investing activities									(77,842)	(80,770)	(86,578)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									11,070	9,078	7,392
Excess tax benefit associated with affiliate’s stock plans									0	95	103
Repurchase of common stock									(383)	0	0
Dividends paid to stockholders (affiliated $(9,182), $(8,162) and $(7,377))									(16,196)	(14,174)	(12,588)
Net cash used in financing activities									(5,509)	(5,001)	(5,093)
Net increase (decrease) in cash									83	(159)	144
Cash at the beginning of the year				224				383	224	383	239
Cash at the end of the year	307				224				307	224	383
Income taxes recovered									(13,967)	(20,254)	(8,703)
Interest paid									337	337	384
Parent company [Member]
ASSETS
Common stock of subsidiaries (equity method)												540,249	514,309
Equity securities available-for-sale, at fair value (cost $2,000 and $0)												2,000	0
Short-term investments												10,874	9,761
Total investments												553,123	524,070
Cash	184			136	136			274	136	274	149	184	136	$ 274	$ 149
Accrued investment income												3	0
Prepaid assets												154	95
Accounts receivable												0	58
Income taxes recoverable												700	683
Total assets												554,164	525,042
LIABILITIES
Accounts payable												46	87
Amounts due affiliate to settle inter-company transaction balances												740	17
Deferred income taxes												36	0
Total liabilities												822	104
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 21,222,535 shares in 2016 and 20,780,439 shares in 2015												21,223	20,781
Additional paid-in capital												119,054	108,747
Accumulated other comprehensive income												46,081	58,433
Retained earnings												366,984	336,977
Total stockholders' equity												553,342	524,938
Total liabilities and stockholders' equity												$ 554,164	$ 525,042
REVENUES
Dividends received from subsidiaries									9,707	9,180	378
Investment income (loss)									13	(9)	(12)
Total revenues									9,720	9,171	366
Other expenses									2,006	1,942	1,584
Income (loss) before income tax benefit and equity in undistributed net income of subsidiaries									7,714	7,229	(1,218)
Income Tax Expense (Benefit)									(698)	(682)	(558)
Income (loss) before equity in undistributed net income of subsidiaries									8,412	7,911	(660)
Equity in undistributed net income of subsidiaries									37,791	42,251	30,652
Net income									46,203	50,162	29,992
Condensed Statements of Comprehensive Income [Abstract]
Net income									46,203	50,162	29,992
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income taxes									(3,885)	(13,037)	34,663
Reclassification adjustment for realized investment gains included in net income, net of income taxes									(6,736)	(4,956)	(4,677)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income taxes:
Net actuarial loss									1,549	863	375
Prior service credit									(2,399)	(2,150)	(2,149)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(850)	(1,287)	(1,774)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial loss									(542)	(3,637)	(5,525)
Prior service cost									(339)	(312)	(35)
Total change in funded status of affiliate's pension and postretirement benefit plans									(881)	(3,949)	(5,560)
Other comprehensive income (loss)									(12,352)	(23,229)	22,652
Total comprehensive income									33,851	26,933	52,644
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities									9,170	7,893	(738)
Cash flows from investing activities
Capital contributions to subsidiaries									(500)	0	0
Purchases of equity securities available-for-sale									(2,000)	0	0
Net (purchases) disposals of short-term investments									(1,113)	(3,030)	5,956
Net cash used in investing activities									(3,613)	(3,030)	5,956
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									11,070	9,078	7,392
Excess tax benefit associated with affiliate’s stock plans									0	95	103
Repurchase of common stock									(383)	0	0
Dividends paid to stockholders (affiliated $(9,182), $(8,162) and $(7,377))									(16,196)	(14,174)	(12,588)
Net cash used in financing activities									(5,509)	(5,001)	(5,093)
Net increase (decrease) in cash									48	(138)	125
Cash at the beginning of the year				$ 136				$ 274	136	274	149
Cash at the end of the year	$ 184				$ 136				184	136	274
Income taxes recovered									716	559	481
Interest paid									$ 0	$ 0	$ 0