SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash	$ 30,140	$ 24,271
Short-term investments, at cost which approximates fair value	11,550	9,980
Investments in unconsolidated investee, at equity value	94,967	90,067
Fixed income:
Available-for-sale fixed income	1,760,515	1,672,239
Property and equipment, at cost, net of accumulated depreciation of $1,426 in 2017 and $1,523 in 2016	54,692	55,849
TOTAL ASSETS	3,105,065	2,947,244
Liabilities:
Bonds payable, long-term debt	149,115	148,928
TOTAL LIABILITIES	2,298,223	2,093,646
Shareholders' Equity
Common stock ($1 par value, authorized 100,000,000 shares, issued 67,078,569 shares in 2017 and 66,874,911 shares in 2016, and outstanding 44,148,355 shares in 2017 and 43,944,697 shares in 2016)	674	67,079
Paid-in capital	305,660	233,077
Accumulated other comprehensive earnings	(14,572)	157,919
Retained earnings	908,079	788,522
Deferred compensation	8,354	8,640
Treasury shares at cost (22,930,214 shares in 2017 and 2016)	(401,353)	(401,639)
TOTAL SHAREHOLDERS’ EQUITY	806,842	853,598	$ 823,572	$ 823,469
TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY	3,105,065	2,947,244
PARENT COMPANY
ASSETS
Cash	3,214	204
Short-term investments, at cost which approximates fair value		70
Accounts receivable, affiliates		1,057
Investments in subsidiaries, at equity value	828,806	912,515
Investments in unconsolidated investee, at equity value	79,521	77,720
Fixed income:
Available-for-sale fixed income	59,878	23,210
Property and equipment, at cost, net of accumulated depreciation of $1,426 in 2017 and $1,523 in 2016	1,914	1,982
Income taxes receivable - current		1,542
Other assets	547	156
TOTAL ASSETS	973,880	1,018,456
Liabilities:
Accounts payable, affiliates	130
Income taxes payable--current	32
Income taxes payable--deferred	15,081	13,207
Bonds payable, long-term debt	149,115	148,928
Interest payable, long-term debt	2,153	2,153
Other liabilities	527	570
TOTAL LIABILITIES	167,038	164,858
Shareholders' Equity
Common stock ($1 par value, authorized 100,000,000 shares, issued 67,078,569 shares in 2017 and 66,874,911 shares in 2016, and outstanding 44,148,355 shares in 2017 and 43,944,697 shares in 2016)	674	67,079
Paid-in capital	305,660	233,077
Accumulated other comprehensive earnings	(14,572)	157,919
Retained earnings	908,079	788,522
Deferred compensation	8,354	8,640
Treasury shares at cost (22,930,214 shares in 2017 and 2016)	(401,353)	(401,639)
TOTAL SHAREHOLDERS’ EQUITY	806,842	853,598
TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY	$ 973,880	$ 1,018,456