Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net income	$ 222,606	$ 157,331	$ 88,497
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on the sale of investments, net	(1,934)	(2,554)	(925)
Depreciation and amortization	4,064	3,219	2,464
Amortization of discount on payments due under Asset Purchase Agreement	0	0	51
Stock-based compensation expense	16,881	13,633	12,520
Amortization of premium on investment securities	10,486	10,004	6,840
Deferred income tax provision	57,564	52,157	44,614
Excess tax benefits from stock-based compensation	(1,083)	(2,420)	(1,809)
Change in:
Accrued investment income	(1,720)	(2,020)	(3,770)
Accounts receivable	(4,955)	(2,828)	(3,803)
Deferred policy acquisition costs	(1,871)	(1,932)	(3,424)
Prepaid federal income tax	(61,860)	(59,739)	(51,673)
Other assets	(526)	(724)	78
Reserve for losses and LAE	10,382	9,333	5,357
Unearned premium reserve	18,571	44,097	53,549
Other accrued liabilities	6,871	6,582	5,138
Net cash provided by operating activities	273,476	224,139	153,704
Investing Activities
Net change in short-term investments	(46,352)	124,692	(196,609)
Purchase of investments available for sale	(656,768)	(798,891)	(699,324)
Proceeds from maturity of investments available for sale	27,186	8,613	27,382
Proceeds from sales of investments available for sale	313,780	449,834	144,744
Purchase of property and equipment	(3,162)	(4,960)	(3,893)
Net cash used in investing activities	(365,316)	(220,712)	(727,700)
Financing Activities
Revolving credit facility borrowings	100,000	0	0
Treasury stock acquired	(4,024)	(5,168)	(2,498)
Payment of issuance costs for revolving credit facility	(2,436)	0	0
Excess tax benefits from stock-based compensation	1,083	2,420	1,809
Issuance of common shares net of costs	0	(537)	126,441
Payments under Asset Purchase Agreement	0	0	(5,000)
Other financing activities	142	53	0
Net cash provided by (used in) financing activities	94,765	(3,232)	120,752
Net increase (decrease) in cash	2,925	195	(453,244)
Cash at beginning of year	24,606	24,411	477,655
Cash at end of year	27,531	24,606	24,411
Supplemental Disclosure of Cash Flow Information
Income tax payments	(30,800)	(15,500)	(1,000)
Interest payments	(324)	0	0
Noncash Transactions
Issuance of management incentive shares (see Note 10)	$ 0	$ 0	$ 416