Schedule II - Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Short-term investments												$ 38,599	$ 53,262
Cash	$ 224			$ 383	$ 383			$ 239	$ 383	$ 239	$ 330	224	383	$ 239	$ 330
Accounts receivable												804	1,767
Income taxes recoverable												1,735	0
Total assets												2,055,437	1,994,576
LIABILITIES
Amounts due affiliate to settle inter-company transaction balances												6,408	8,559
Total liabilities												1,011,017	994,934
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 20,780,439 shares in 2015 and 20,344,409 shares in 2014												20,781	20,344
Additional paid-in capital												108,747	99,891
Accumulated other comprehensive income												58,433	81,662	59,010
Retained earnings												336,977	300,989
Total stockholders' equity												524,938	502,886	455,210	401,209
Total liabilities and stockholders' equity												1,535,955	1,497,820
REVENUES
Investment loss									45,582	46,465	43,022
Total revenues	147,479	$ 165,104	$ 158,808	152,335	149,844	$ 150,659	$ 147,733	146,231	623,726	594,467	567,985
Other expenses									2,690	2,377	2,115
Income Tax Expense (Benefit)	4,028	4,732	3,127	9,607	7,711	(346)	(744)	4,294	21,494	10,915	17,334
Net income	9,895	11,189	8,748	20,330	16,154	2,229	1,014	10,595	50,162	29,992	43,519
Condensed Statements of Comprehensive Income [Abstract]
Net income	9,895	$ 11,189	$ 8,748	20,330	16,154	$ 2,229	$ 1,014	10,595	50,162	29,992	43,519
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income taxes									(13,037)	34,663	(15,582)
Reclassification adjustment for realized investment gains included in net income, net of income taxes									(4,956)	(4,677)	(5,848)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit cost (income), net of deferred income taxes:
Net actuarial loss									863	375	1,882
Prior service credit									(2,150)	(2,149)	(460)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(1,287)	(1,774)	1,422
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)									(3,637)	(5,525)	13,718
Prior service (cost) credit									(312)	(35)	17,548
Total change in funded status of affiliate's pension and postretirement benefit plans									(3,949)	(5,560)	31,266
Other comprehensive income (loss)									(23,229)	22,652	11,258
Total comprehensive income									26,933	52,644	54,777
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities									85,612	91,815	86,833
Cash flows from investing activities
Net (purchases) sales of short-term investments									14,663	2,904	(2,786)
Net cash (used in) provided by investing activities									(80,770)	(86,578)	(85,953)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									9,078	7,392	10,208
Excess tax benefit associated with affiliate’s stock plans									95	103	96
Dividends paid to stockholders (affiliated $(8,162), $(7,377) and $(6,749))									(14,174)	(12,588)	(11,275)
Net cash used in financing activities									(5,001)	(5,093)	(971)
Net increase (decrease) in cash									(159)	144	(91)
Cash at the beginning of the year				383				239	383	239	330
Cash at the end of the year	224				383				224	383	239
Income taxes recovered									20,254	8,703	13,714
Interest paid									337	384	900
Parent company [Member]
ASSETS
Investment in common stock of subsidiaries (equity method)												514,309	495,288
Short-term investments												9,761	6,731
Cash	136			274	274			149	274	149	291	136	274	$ 149	$ 291
Prepaid assets												95	87
Accounts receivable												58	69
Income taxes recoverable												683	559
Total assets												525,042	503,008
LIABILITIES
Accounts payable												87	80
Amounts due affiliate to settle inter-company transaction balances												17	42
Total liabilities												104	122
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 30,000,000 shares; issued and outstanding, 20,780,439 shares in 2015 and 20,344,409 shares in 2014												20,781	20,344
Additional paid-in capital												108,747	99,891
Accumulated other comprehensive income												58,433	81,662
Retained earnings												336,977	300,989
Total stockholders' equity												524,938	502,886
Total liabilities and stockholders' equity												$ 525,042	$ 503,008
REVENUES
Dividends received from subsidiaries									9,180	378	9,974
Investment loss									(9)	(12)	(10)
Total revenues									9,171	366	9,964
Other expenses									1,942	1,584	1,364
Income (loss) before income tax benefit and equity in undistributed net income of subsidiaries									7,229	(1,218)	8,600
Income Tax Expense (Benefit)									(682)	(558)	(481)
Income (loss) before equity in undistributed net income of subsidiaries									7,911	(660)	9,081
Equity in undistributed net income of subsidiaries									42,251	30,652	34,438
Net income									50,162	29,992	43,519
Condensed Statements of Comprehensive Income [Abstract]
Net income									50,162	29,992	43,519
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income taxes									(13,037)	34,663	(15,582)
Reclassification adjustment for realized investment gains included in net income, net of income taxes									(4,956)	(4,677)	(5,848)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit cost (income), net of deferred income taxes:
Net actuarial loss									863	375	1,882
Prior service credit									(2,150)	(2,149)	(460)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(1,287)	(1,774)	1,422
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)									(3,637)	(5,525)	13,718
Prior service (cost) credit									(312)	(35)	17,548
Total change in funded status of affiliate's pension and postretirement benefit plans									(3,949)	(5,560)	31,266
Other comprehensive income (loss)									(23,229)	22,652	11,258
Total comprehensive income									26,933	52,644	54,777
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities									7,893	(738)	9,544
Cash flows from investing activities
Net (purchases) sales of short-term investments									(3,030)	5,956	(8,715)
Net cash (used in) provided by investing activities									(3,030)	5,956	(8,715)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans									9,078	7,392	10,208
Excess tax benefit associated with affiliate’s stock plans									95	103	96
Dividends paid to stockholders (affiliated $(8,162), $(7,377) and $(6,749))									(14,174)	(12,588)	(11,275)
Net cash used in financing activities									(5,001)	(5,093)	(971)
Net increase (decrease) in cash									(138)	125	(142)
Cash at the beginning of the year				$ 274				$ 149	274	149	291
Cash at the end of the year	$ 136				$ 274				136	274	149
Income taxes recovered									559	481	0
Interest paid									$ 0	$ 0	$ 0