Schedule II - Condensed Financial Information of Registrant (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
ASSETS
Short-term investments	$ 53,262								$ 56,166								$ 53,262		$ 56,166
Cash	383								239								383		239		330
Accounts receivable	1,767								1,080								1,767		1,080
Total assets	1,994,576								1,816,966								1,994,576		1,816,966
LIABILITIES
Amounts due affiliate to settle inter-company transaction balances	8,559								9,090								8,559		9,090
Total liabilities	994,934								919,291								994,934		919,291
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 20,000,000 shares; issued and outstanding, 13,562,980 shares in 2014 and 13,306,027 shares in 2013	13,563								13,306								13,563		13,306
Additional paid-in capital	106,672								99,309								106,672		99,309
Accumulated other comprehensive income	81,662	[1]							59,010	[1]							81,662	[1]	59,010	[1]	47,752	[1]
Retained earnings	300,989								283,585								300,989		283,585
Total stockholders' equity	502,886								455,210								502,886		455,210		401,209		352,341
Total liabilities and stockholders' equity	1,497,820								1,374,501								1,497,820		1,374,501
REVENUES
Investment loss																	46,465		43,022		44,145
Total revenues	149,844	[2]	150,659	[2]	147,733	[2]	146,231	[2]	151,683	[2]	143,552	[2]	138,466	[2]	134,284	[2]	594,467		567,985		511,842
Other expenses																	2,377		2,115		2,097
Income Tax Expense (Benefit)	7,711		(346)		(744)		4,294		6,996		2,365		1,737		6,236		10,915		17,334		13,667
Net income	16,154		2,229		1,014		10,595		15,835		7,199		6,212		14,273		29,992		43,519		37,966
Condensed Statements of Comprehensive Income [Abstract]
Net income	16,154		2,229		1,014		10,595		15,835		7,199		6,212		14,273		29,992		43,519		37,966
OTHER COMPREHENSIVE INCOME (LOSS)
Change in unrealized holding gains (losses) on investment securities, net of deferred income taxes																	34,663		(15,582)		23,863
Reclassification adjustment for realized investment gains included in net income, net of income taxes																	(4,677)		(5,848)		(5,211)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit cost (income), net of deferred income taxes:
Net actuarial loss																	375		1,882		2,108
Prior service credit																	(2,149)		(460)		(344)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans																	(1,774)		1,422		1,764
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)																	(5,525)		13,718		(736)
Prior service (cost) credit																	(35)		17,548		732
Total change in funded status of affiliate's pension and postretirement benefit plans																	(5,560)		31,266		(4)
Other comprehensive income																	22,652	[1]	11,258	[1]	20,412
Total comprehensive income																	52,644		54,777		58,378
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities																	91,815		86,833		55,038
Cash flows from investing activities
Net (purchases) sales of short-term investments																	2,904		(2,786)		(10,790)
Net cash (used in) provided by investing activities																	(86,578)		(85,953)		(45,213)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans																	7,392		10,208		692
Excess tax benefit associated with affiliate’s stock plans																	103		96		(3)
Dividends paid to stockholders (affiliated $(7,377), $(6,749) and $(6,357))																	(12,588)		(11,275)		(10,439)
Net cash used in financing activities																	(5,093)		(971)		(9,750)
Net increase (decrease) in cash																	144		(91)		75
Cash at the beginning of the year							239								330		239		330		255
Cash at the end of the year	383								239								383		239		330
Income taxes recovered																	(8,703)		(13,714)		(3,514)
Interest paid																	384		900		900
Parent company [Member]
ASSETS
Investment in common stock of subsidiaries (equity method)	495,288								441,984								495,288		441,984
Short-term investments	6,731								12,687								6,731		12,687
Cash	274								149								274		149		291
Prepaid assets	87								67								87		67
Accounts receivable	69								0								69		0
Income taxes recoverable	559								481								559		481
Total assets	503,008								455,368								503,008		455,368
LIABILITIES
Accounts payable	80								101								80		101
Amounts due affiliate to settle inter-company transaction balances	42								57								42		57
Total liabilities	122								158								122		158
STOCKHOLDERS' EQUITY
Common stock, $1 par value, authorized 20,000,000 shares; issued and outstanding, 13,562,980 shares in 2014 and 13,306,027 shares in 2013	13,563								13,306								13,563		13,306
Additional paid-in capital	106,672								99,309								106,672		99,309
Accumulated other comprehensive income	81,662								59,010								81,662		59,010
Retained earnings	300,989								283,585								300,989		283,585
Total stockholders' equity	502,886								455,210								502,886		455,210
Total liabilities and stockholders' equity	503,008								455,368								503,008		455,368
REVENUES
Dividends received from subsidiaries																	378		9,974		12,050
Investment loss																	(12)		(10)		(10)
Total revenues																	366		9,964		12,040
Other expenses																	1,584		1,364		1,299
Income (loss) before income tax benefit and equity in undistributed net income of subsidiaries																	(1,218)		8,600		10,741
Income Tax Expense (Benefit)																	(558)		(481)		(462)
Income (loss) before equity in undistributed net income of subsidiaries																	(660)		9,081		11,203
Equity in undistributed net income of subsidiaries																	30,652		34,438		26,763
Net income																	29,992		43,519		37,966
Condensed Statements of Comprehensive Income [Abstract]
Net income																	29,992		43,519		37,966
OTHER COMPREHENSIVE INCOME (LOSS)
Change in unrealized holding gains (losses) on investment securities, net of deferred income taxes																	34,663		(15,582)		23,863
Reclassification adjustment for realized investment gains included in net income, net of income taxes																	(4,677)		(5,848)		(5,211)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit cost (income), net of deferred income taxes:
Net actuarial loss																	375		1,882		2,108
Prior service credit																	(2,149)		(460)		(344)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans																	(1,774)		1,422		1,764
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income taxes:
Net actuarial gain (loss)																	(5,525)		13,718		(736)
Prior service (cost) credit																	(35)		17,548		732
Total change in funded status of affiliate's pension and postretirement benefit plans																	(5,560)		31,266		(4)
Other comprehensive income																	22,652	[1]	11,258		20,412
Total comprehensive income																	52,644		54,777		58,378
Condensed Statements of Cash Flows [Abstract]
Net cash provided by (used in) operating activities																	(738)		9,544		11,117
Cash flows from investing activities
Net (purchases) sales of short-term investments																	5,956		(8,715)		(1,198)
Net cash (used in) provided by investing activities																	5,956		(8,715)		(1,198)
Cash flows from financing activities
Issuance of common stock through affiliate’s stock plans																	7,392		10,208		692
Excess tax benefit associated with affiliate’s stock plans																	103		96		(3)
Dividends paid to stockholders (affiliated $(7,377), $(6,749) and $(6,357))																	(12,588)		(11,275)		(10,439)
Net cash used in financing activities																	(5,093)		(971)		(9,750)
Net increase (decrease) in cash																	125		(142)		169
Cash at the beginning of the year							149								291		149		291		122
Cash at the end of the year	274								149								274		149		291
Income taxes recovered																	481		0		471
Interest paid																	$ 0		$ 0		$ 0

[1] All amounts are net of tax. Amounts in parentheses indicate debits.

[2] Amounts include the reclassification of foreign currency exchange rate gains (losses) to other income in the consolidated financial statements. Prior to the fourth quarter of 2014, these amounts were reported as a component of other expenses.