SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash	$ 24,271	$ 18,269	$ 11,081	$ 30,620
Short-term investments, at cost which approximates fair value	9,980	5,015
Investments in unconsolidated investee, at equity value	90,067	72,240
Fixed income:
Available-for-sale, at fair value (amortized cost - $23,184 in 2017 and $45,901 in 2016)	1,672,239	1,605,209
Property and equipment, at cost, net of accumulated depreciation of $1,426 in 2017 and $1,523 in 2016	55,849	54,606
TOTAL ASSETS	2,947,244	2,777,633
Liabilities:
Bonds payable, long-term debt	148,928	148,741
TOTAL LIABILITIES	2,093,646	1,954,061
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)	67,079	66,875
Paid-in capital	233,077	229,779
Accumulated other comprehensive earnings	157,919	122,610
Retained earnings	788,522	797,307
Deferred compensation	8,640	11,496
Treasury shares at cost (22,930,214 shares in 2017 and 2016)	(401,639)	(404,495)
TOTAL SHAREHOLDERS’ EQUITY	853,598	823,572	823,469	845,062
TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY	2,947,244	2,777,633
PARENT COMPANY
ASSETS
Cash	204	21	$ 178	$ 374
Short-term investments, at cost which approximates fair value	70	23
Accounts receivable, affiliates	1,057	127
Investments in subsidiaries, at equity value	912,515	882,095
Investments in unconsolidated investee, at equity value	77,720	62,604
Fixed income:
Available-for-sale, at fair value (amortized cost - $23,184 in 2017 and $45,901 in 2016)	23,210	45,885
Property and equipment, at cost, net of accumulated depreciation of $1,426 in 2017 and $1,523 in 2016	1,982	2,213
Income taxes receivable - current	1,542	858
Other assets	156	329
TOTAL ASSETS	1,018,456	994,155
Liabilities:
Income taxes payable--deferred	13,207	19,145
Bonds payable, long-term debt	148,928	148,741
Interest payable, long-term debt	2,153	2,153
Other liabilities	570	544
TOTAL LIABILITIES	164,858	170,583
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)	67,079	66,875
Paid-in capital	233,077	229,779
Accumulated other comprehensive earnings	157,919	122,610
Retained earnings	788,522	797,307
Deferred compensation	8,640	11,496
Treasury shares at cost (22,930,214 shares in 2017 and 2016)	(401,639)	(404,495)
TOTAL SHAREHOLDERS’ EQUITY	853,598	823,572
TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY	$ 1,018,456	$ 994,155