Schedule II - Condensed Financial Information of Registrant (Condensed Statements of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating activities
Net income	$ 253,283	$ 57,798	$ 86,248
Adjustments to reconcile net income to net cash provided by operating activities:
Change in fair value of 2015 notes embedded conversion derivative	133,968	286,899	(105,194)
Provision for depreciation and amortization	18,375	18,404	18,970
Amortization of premium on fixed maturity security	6,861	(69,828)	(154,483)
Change in fair value of derivatives	(1,076,015)	(221,138)	112,608
Realized gain on investments	(40,561)	6,454	18,641
Loss on extinguishment of debt	32,515	0	0
Share-based compensation	10,476	6,552	9,339
ESOP compensation	3,500	1,100	1,300
Deferred income taxes (benefits)	3,013	(52,336)	(80,869)
Other	(7,059)	(857)	59
Changes in operating assets and liabilities:
Other assets	1,113	(7,090)	5,005
Other liabilities	(11,435)	5,927	(77,082)
Net cash provided by operating activities	862,668	784,314	277,791
Investing activities
Net cash provided by (used in) investing activities	(3,879,296)	(2,129,490)	(3,605,833)
Financing activities
Financing fees incurred and deferred	(11,942)	0	(1,566)
Repayment of notes payable	(234,154)	0	(46,251)
Net proceeds from settlement of notes hedge and warrants	22,170	0	0
Repayment of subordinated debentures	0	(1,141)	0
Excess tax benefits realized from share-based compensation plans	4,043	481	1,061
Proceeds from issuance of common stock	31,764	5,741	4,686
Dividends paid	(12,643)	(9,203)	(6,916)
Net cash provided by (used in) financing activities	2,645,612	2,208,769	3,135,228
Increase (decrease) in cash and cash equivalents	(371,016)	863,593	(192,814)
Cash and cash equivalents at beginning of year	1,268,545	404,952	597,766
Cash and cash equivalents at end of year	897,529	1,268,545	404,952
Supplemental disclosures of cash flow information
Interest paid	25,608	27,666	30,650
Non-cash financing activity:
Conversion of subordinated debentures	0	20,770	0
Common stock issued in extinguishment of debt	117,463	0	0
Parent Company [Member]
Operating activities
Net income	253,283	57,798	86,248
Adjustments to reconcile net income to net cash provided by operating activities:
Change in fair value of 2015 notes embedded conversion derivative	141,974	(2,488)	(21,002)
Provision for depreciation and amortization	2,831	2,382	2,395
Accrual of discount on equity security	(5)	(5)	(5)
Equity in undistributed income of subsidiaries	(279,961)	(65,777)	(97,314)
Amortization of premium on fixed maturity security	0	0	1,005
Accrual of discount on contingent convertible notes	12,417	12,261	13,024
Change in fair value of derivatives	(144,012)	7,472	21,002
Realized gain on investments	0	0	(18)
Loss on extinguishment of debt	32,515	0	0
Accrual of discount on debenture issued to subsidiary trust	181	169	158
Share-based compensation	1,407	1,348	866
ESOP compensation	110	45	45
Deferred income taxes (benefits)	(5,202)	(2,838)	(4,355)
Other	(3,608)	0	0
Changes in operating assets and liabilities:
Receivable from subsidiaries	995	1,205	1,596
Federal income tax recoverable	62	(2,745)	142
Other assets	(1,945)	(549)	1,702
Other liabilities	20,131	(4,528)	(1,562)
Net cash provided by operating activities	31,173	3,750	3,927
Investing activities
Capital contributions to subsidiaries	0	0	(2,450)
Purchase of fixed maturity security	0	0	(53,610)
Sales, maturities or repayments of fixed maturity securities—available for sale	0	0	52,623
Net cash provided by (used in) investing activities	0	0	(3,437)
Financing activities
Financing fees incurred and deferred	(11,942)	0	(1,566)
Proceeds from notes payable	415,000	0	0
Repayment of notes payable	(234,154)	0	(46,251)
Net proceeds from settlement of notes hedge and warrants	22,170	0	0
Repayment of subordinated debentures	0	(1,141)	0
Excess tax benefits realized from share-based compensation plans	159	6	28
Proceeds from issuance of common stock	31,764	5,370	4,686
Dividends paid	(12,849)	(9,374)	(7,102)
Net cash provided by (used in) financing activities	210,148	(5,139)	(50,205)
Increase (decrease) in cash and cash equivalents	241,321	(1,389)	(49,715)
Cash and cash equivalents at beginning of year	11,220	12,609	62,324
Cash and cash equivalents at end of year	252,541	11,220	12,609
Non-cash financing activity:
Conversion of subordinated debentures	0	20,770	0
Common stock issued in extinguishment of debt	117,463	0	0
Parent Company [Member] | Convertible Senior Notes [Member]
Supplemental disclosures of cash flow information
Interest paid	13,758	14,564	16,917
Parent Company [Member] | Convertible Subordinated Debentures [Member]
Supplemental disclosures of cash flow information
Interest paid	$ 11,850	$ 13,102	$ 13,703