Condensed Financial Information - Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities:
Net income	$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 19,275	$ 31,225	$ 16,861	$ 27,968	$ 81,057	$ 95,329	$ 63,466
Adjustments to reconcile net income to net cash provided by (used in) operations:
Call premium on redemption of Senior notes											17,895
Other									(4,679)	29,039	31,987
Net cash provided by (used in) operating activities									227,624	222,492	136,875
Fixed maturities, available-for-sale
Sales									376,522	362,136	648,335
Purchases									(648,059)	(864,902)	(899,930)
Equity securities
Sales									96,405	54,900	72,113
Purchases									(215,405)	(83,845)	(89,288)
Net increase in short-term investments									(38,468)	117,740	(142,214)
Net cash provided by (used in) investing activities									(249,855)	(219,470)	(229,859)
Financing activities:
Net proceeds from debt offering											263,278
Redemption of 7.0% Senior notes due May 1, 2016											(132,437)
Proceeds of stock issued from employee stock purchase plan									1,352	1,067	821
Proceeds of stock issued from exercise of stock options									29	153	2,495
Net cash provided by (used in) financing activities									1,381	1,220	134,157
Increase (decrease) in cash									(20,850)	4,242	41,173
Cash at beginning of year				90,751				86,509	90,751	86,509	45,336
Cash at end of period	69,901				90,751				69,901	90,751	86,509
Parent Company
Operating activities:
Net income									81,057	95,329	63,466
Adjustments to reconcile net income to net cash provided by (used in) operations:
Equity in undistributed net income of wholly-owned subsidiaries									(90,862)	(105,379)	(81,971)
Call premium on redemption of Senior notes											17,895
Other									3,373	9,211	2,098
Net cash provided by (used in) operating activities									(6,432)	(839)	1,488
Fixed maturities, available-for-sale
Sales									1,250	3,200	8,754
Purchases											(1,249)
Equity securities
Sales									0	0	0
Purchases									0	0	0
Net increase in short-term investments									3,918	(3,424)	(89,988)
Net cash provided by (used in) investing activities									5,168	(224)	(82,483)
Financing activities:
Capital contribution to subsidiary											(50,000)
Net proceeds from debt offering											263,278
Redemption of 7.0% Senior notes due May 1, 2016											(132,437)
Proceeds of stock issued from employee stock purchase plan									1,352	1,067	821
Proceeds of stock issued from exercise of stock options									29	153	2,495
Net cash provided by (used in) financing activities									1,381	1,220	84,157
Increase (decrease) in cash									117	157	3,162
Cash at beginning of year				$ 6,300				$ 6,143	6,300	6,143	2,981
Cash at end of period	$ 6,417				$ 6,300				$ 6,417	$ 6,300	$ 6,143