CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income	$ 328,447,000	$ 235,030,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	48,882,000	44,324,000
Deferred tax expense	165,250,000	111,191,000
Net realized investment losses (gains)	211,000	(8,984,000)
Loss on debt extinguishment	65,000	90,531,000
Change in certain assets and liabilities:
Accrued investment income	1,145,000	(2,086,000)
Reinsurance recoverable on loss reserves	4,615,000	(2,376,000)
Reinsurance recoverable on paid losses	326,000	(1,313,000)
Premium receivable	(1,546,000)	1,048,000
Deferred insurance policy acquisition costs	(1,265,000)	(2,167,000)
Profit commission receivable	(3,899,000)	(2,005,000)
Loss reserves	(333,662,000)	(357,919,000)
Unearned premiums	41,079,000	41,353,000
Return premium accrual	(18,000,000)	(12,800,000)
Income taxes payable - current	33,943,000	822,000
Other, net	(9,235,000)	14,337,000
Net cash provided by operating activities	256,356,000	148,986,000
Purchases of investments:
Fixed income securities	(774,985,000)	(1,105,995,000)
Equity securities	(58,000)	(4,315,000)
Proceeds from sales of fixed income securities	233,198,000	718,894,000
Proceeds from maturity of fixed income securities	547,699,000	432,557,000
Proceeds from sale of equity securities	0	6,425,000
Net increase in payable for securities	3,738,000	3,376,000
Additions to property and equipment	(12,121,000)	(4,969,000)
Net cash (used in) provided by investing activities	(2,529,000)	45,973,000
Cash flows from financing activities:
Proceeds from revolving credit facility	150,000,000	0
Repayment of revolving credit facility	(150,000,000)	0
Proceeds from issuance of long-term debt	0	573,094,000
Purchase or repayment of convertible senior notes	(145,620,000)	(363,778,000)
Payment of original issue discount - convertible senior notes	(4,504,000)	(11,250,000)
Purchase of convertible junior subordinated debentures	0	(100,860,000)
Payment of original issue discount - convertible junior subordinated debentures	0	(41,540,000)
Repurchase of common stock	0	(91,597,000)
Cash portion of loss on debt extinguishment	0	(59,460,000)
Payment of debt issuance costs	(1,630,000)	(938,000)
Payment of withholding taxes related to share-based compensation net share settlement	(6,782,000)	(5,007,000)
Net cash used in financing activities	(158,536,000)	(101,336,000)
Net increase in cash and cash equivalents	95,291,000	93,623,000
Cash and cash equivalents at beginning of period	155,410,000	181,120,000
Cash and cash equivalents at end of period	$ 250,701,000	$ 274,743,000