Schedule II. Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Equity securities	$ 9,282	$ 33
Short-term investments	25,770	29,580
Other investments		13,330
Total investments	1,130,314	1,084,474
Cash and cash equivalents	55,559	58,936	$ 69,481	$ 90,956
Deferred income taxes	19,262	33,811
Property and equipment, net	6,128	6,636
Federal income tax recoverable	1,761
Other assets	9,959	37,668
Total assets	1,518,236	1,518,856
Liabilities, redeemable preferred stock and shareholders’ equity
Total liabilities	1,092,813	1,062,706
Shareholders' equity (net of Treasury stock of $22,370 at December 31, 2017 and 2016)	425,423	456,150	453,981	446,968
Total liabilities and shareholders’ equity	1,518,236	1,518,856
Parent Company [Member]
Assets
Equity securities	9,248
Short-term investments	7,962	11,939
Other investments		13,330
Investment in subsidiaries	386,844	415,767
Total investments	404,054	441,036
Cash and cash equivalents	16,351	9,596	$ 26,968	$ 43,585
Deferred income taxes	302	675
Notes receivable from subsidiaries	1,711	1,587
Property and equipment, net	1,359	1,942
Federal income tax recoverable	3,525	3,091
Other assets	887	657
Total assets	428,189	458,584
Liabilities, redeemable preferred stock and shareholders’ equity
Total liabilities	2,766	2,434
Shareholders' equity (net of Treasury stock of $22,370 at December 31, 2017 and 2016)	425,423	456,150
Total liabilities and shareholders’ equity	$ 428,189	$ 458,584