Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net cash (used in) provided by operating activities	$ 140,440	$ 128,903	$ 81,048
Investing activities
Purchases of property and equipment	(989)	(1,140)	(1,196)
Net cash used in investing activities	(141,008)	(96,960)	(9,330)
Financing activities
Proceeds from stock option exercises	2,674	2,344	781
Tax benefit from share-based payments	2,841	2,024	421
Redemption of Subordinated Debt Security			(25,780)
Dividends to shareholders	(37,068)	(5,910)
Net cash used in financing activities	(31,553)	(1,542)	(24,578)
Change in cash and cash equivalents	(32,121)	30,401	47,140
Cash and cash equivalents at beginning of year	123,077	92,676	45,536
Cash and cash equivalents at end of year	90,956	123,077	92,676
Parent Company [Member]
Operating activities
Net cash (used in) provided by operating activities	2,161	1,829	5,149
Investing activities
Purchases of investments	(1,462)	(10,248)	(23,361)
Proceeds from sales of investments	2,404	11,674	39,537
Purchases of property and equipment	(691)	(779)	(842)
Dividends from subsidiary	25,000	15,000	21,000
Net cash used in investing activities	25,251	15,647	36,334
Financing activities
Proceeds from stock option exercises	2,674	2,344	781
Tax benefit from share-based payments	2,841	2,024	421
Redemption of Subordinated Debt Security			(25,780)
Dividends to shareholders	(37,068)	(5,910)
Net cash used in financing activities	(31,553)	(1,542)	(24,578)
Change in cash and cash equivalents	(4,141)	15,934	16,905
Cash and cash equivalents at beginning of year	47,726	31,792	14,887
Cash and cash equivalents at end of year	$ 43,585	$ 47,726	$ 31,792