Condensed Financial Information - Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities:
Net income	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 82,726	$ 81,057	$ 95,329
Adjustments to reconcile net income to net cash provided by (used in) operations:
Other									22,593	(19,676)	17,532
Net cash provided by (used in) operating activities									224,227	227,624	222,492
Fixed maturities, available-for-sale
Sales									446,666	376,522	362,136
Purchases									(962,419)	(648,059)	(864,902)
Equity securities
Sales									54,798	96,405	54,900
Purchases									(92,329)	(215,405)	(83,845)
Net (increase) decrease in short-term investments									73,541	(38,468)	117,740
Net cash provided by (used in) investing activities									(211,432)	(249,855)	(219,470)
Financing activities:
Proceeds of stock issued from employee stock purchase plan									1,840	1,352	1,067
Proceeds of stock issued from exercise of stock options										29	153
Dividends paid									(3,930)
Net cash provided by (used in) financing activities									(2,090)	1,381	1,220
Change in cash									(5,258)	(20,850)	4,242
Cash at beginning of year				69,901				90,751	69,901	90,751	86,509
Cash at end of period	64,643				69,901				64,643	69,901	90,751
Parent Company
Adjustments to reconcile net income to net cash provided by (used in) operations:
Equity in undistributed net income of wholly-owned subsidiaries									(90,121)	(90,862)	(105,379)
Dividends received from subsidiaries									5,000
Other									379	3,373	9,211
Net cash provided by (used in) operating activities									(2,016)	(6,432)	(839)
Fixed maturities, available-for-sale
Sales										1,250	3,200
Equity securities
Sales									0	0	0
Purchases									0	0	0
Net (increase) decrease in short-term investments									87,942	3,918	(3,424)
Net cash provided by (used in) investing activities									87,942	5,168	(224)
Financing activities:
Capital contribution to subsidiary									(79,250)
Proceeds of stock issued from employee stock purchase plan									1,840	1,352	1,067
Proceeds of stock issued from exercise of stock options										29	153
Dividends paid									(3,930)
Net cash provided by (used in) financing activities									(81,340)	1,381	1,220
Change in cash									4,586	117	157
Cash at beginning of year				$ 6,417				$ 6,300	6,417	6,300	6,143
Cash at end of period	$ 11,003				$ 6,417				$ 11,003	$ 6,417	$ 6,300