Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net cash (used in) provided by operating activities	$ 114,212	$ 92,859	$ 140,684
Investing activities
Purchases of property and equipment	(1,638)	(953)	(989)
Net cash (used in) provided by investing activities	(50,478)	(49,820)	(141,252)
Financing activities
Proceeds from stock option exercises	837	1,844	2,674
Tax benefit from share-based payments	995	2,203	2,841
Dividends to shareholders	(76,111)	(68,561)	(37,068)
Net cash used in financing activities	(74,279)	(64,514)	(31,553)
Change in cash and cash equivalents	(10,545)	(21,475)	(32,121)
Cash and cash equivalents at beginning of year	69,481	90,956	123,077
Cash and cash equivalents at end of year	58,936	69,481	90,956
Parent Company [Member]
Operating activities
Net cash (used in) provided by operating activities	2,771	(1,727)	2,161
Investing activities
Purchases of investments	(11,940)		(1,462)
Proceeds from sales of investments		250	2,404
Purchases of property and equipment	(924)	(627)	(691)
Dividends from subsidiary	67,000	50,000	25,000
Net cash (used in) provided by investing activities	54,136	49,623	25,251
Financing activities
Proceeds from stock option exercises	837	1,844	2,674
Tax benefit from share-based payments	995	2,204	2,841
Dividends to shareholders	(76,111)	(68,561)	(37,068)
Net cash used in financing activities	(74,279)	(64,513)	(31,553)
Change in cash and cash equivalents	(17,372)	(16,617)	(4,141)
Cash and cash equivalents at beginning of year	26,968	43,585	47,726
Cash and cash equivalents at end of year	$ 9,596	$ 26,968	$ 43,585