Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net cash (used in) provided by operating activities	$ 130,810	$ 114,212	$ 92,859
Investing activities
Purchases of property and equipment	(478)	(1,638)	(953)
Net cash used in investing activities	(51,542)	(50,478)	(49,820)
Financing activities
Proceeds from stock option exercises		837	1,844
Tax benefit from share-based payments		995	2,203
Dividends to shareholders	(82,645)	(76,111)	(68,561)
Net cash used in financing activities	(82,645)	(74,279)	(64,514)
Change in cash and cash equivalents	(3,377)	(10,545)	(21,475)
Cash and cash equivalents at beginning of year	58,936	69,481	90,956
Cash and cash equivalents at end of year	55,559	58,936	69,481
Parent Company [Member]
Operating activities
Net cash (used in) provided by operating activities	15,138	2,771	(1,727)
Investing activities
Purchases of investments	(37,361)	(11,940)
Proceeds from sales of investments	33,000		250
Purchases of property and equipment	(277)	(924)	(627)
Dividends from subsidiary	78,900	67,000	50,000
Net cash used in investing activities	74,262	54,136	49,623
Financing activities
Proceeds from stock option exercises		837	1,844
Tax benefit from share-based payments		995	2,204
Dividends to shareholders	(82,645)	(76,111)	(68,561)
Net cash used in financing activities	(82,645)	(74,279)	(64,513)
Change in cash and cash equivalents	6,755	(17,372)	(16,617)
Cash and cash equivalents at beginning of year	9,596	26,968	43,585
Cash and cash equivalents at end of year	$ 16,351	$ 9,596	$ 26,968