Schedule II - Condensed Financial Information of Registrant (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Aug. 01, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income		$ 36,803	$ 56,957	$ 26,946	$ 53,939	$ 120,796	$ (7,420)	$ 14,708	$ (44,841)	$ 174,645	$ 83,243	$ 219,830
Adjustments to reconcile net income to net cash provided by operating activities:
Change in fair value of 2015 notes embedded conversion derivative										919,735	543,465	(464,698)
Change in fair value of derivatives										(1,678,956)	(165,727)	334,300
Loss on extinguishment of debt		0	$ 18,389	$ 428	0					18,817	0	0
Share-based compensation										6,464	6,692	7,373
Deferred income taxes										(46,730)	(10,408)	41,916
Changes in operating assets and liabilities:
Other assets										448	1,812	71
Other liabilities										(84,416)	(55,940)	75,794
Net cash provided by operating activities										1,922,393	1,414,322	504,804
Investing activities
Purchases of property, plant and equipment										(4,809)	(1,197)	(1,313)
Net cash used in investing activities										(2,591,936)	(4,499,045)	(5,577,205)
Financing activities
Financing fees incurred and deferred										(5,817)	(1,456)	0
Repayment of notes payable										(413,252)	0	(48,152)
Repayment of loan payable										(100,000)	0	0
Proceeds from issuance of notes payable										499,650	0	0
Proceeds from issuance of loan payable										0	100,000	0
Proceeds from issuance of common stock	$ 134,700									14,028	139,654	112,481
Net proceeds from settlement of notes hedges and warrants										0	0	25,775
Dividends paid										(23,148)	(21,110)	(17,874)
Net cash provided by (used in) financing activities										1,312,322	3,478,240	4,768,636
Increase (decrease) in cash and cash equivalents										642,779	393,517	(303,765)
Cash and cash equivalents at beginning of year					791,266				397,749	791,266	397,749	701,514
Cash and cash equivalents at end of year		1,434,045				791,266				1,434,045	791,266	397,749
Cash paid during the year for interest:
Interest paid										55,445	39,647	39,118
Non-cash financing activity:
Common stock issued to settle warrants that have expired										0	93	48
Parent Company [Member]
Operating activities
Net income										174,645	83,243	219,830
Adjustments to reconcile net income to net cash provided by operating activities:
Change in fair value of 2015 notes embedded conversion derivative										0	0	(4,516)
Provision for depreciation and amortization										1,610	1,946	1,613
Accrual of discount on equity security										(7)	(7)	(6)
Equity in undistributed income of subsidiaries										(165,572)	(65,635)	(209,698)
Accrual of discount on contingent convertible notes										0	0	698
Change in fair value of derivatives										(657)	(698)	6,377
Loss on extinguishment of debt										18,817	0	0
Accrual of discount on debenture issued to subsidiary trust										236	221	207
Share-based compensation										951	818	1,026
Deferred income taxes										1,583	2,117	8,967
Changes in operating assets and liabilities:
Receivable from subsidiaries										16	(125)	93
Federal income tax recoverable/payable										(4,673)	11,361	2,683
Other assets										158	(326)	(4)
Other liabilities										(12,427)	2,546	(1,664)
Net cash provided by operating activities										14,680	35,461	25,606
Investing activities
Capital contributions to subsidiaries										0	(255,000)	(120,000)
Purchases of property, plant and equipment										(45)	(54)	0
Net cash used in investing activities										(45)	(255,054)	(120,000)
Financing activities
Financing fees incurred and deferred										(5,817)	(1,456)	0
Repayment of notes payable										(413,252)	0	(48,152)
Repayment of loan payable										(100,000)	0	0
Proceeds from issuance of notes payable										499,650	0	0
Proceeds from issuance of loan payable										0	100,000	0
Proceeds from issuance of common stock										14,028	139,654	112,481
Net proceeds from settlement of notes hedges and warrants										0	0	25,775
Dividends paid										(23,152)	(21,114)	(17,946)
Net cash provided by (used in) financing activities										(28,543)	217,084	72,158
Increase (decrease) in cash and cash equivalents										(13,908)	(2,509)	(22,236)
Cash and cash equivalents at beginning of year					$ 36,394				$ 38,903	36,394	38,903	61,139
Cash and cash equivalents at end of year		$ 22,486				$ 36,394				22,486	36,394	38,903
Non-cash financing activity:
Common stock issued to settle warrants that have expired										0	93	48
Parent Company [Member] | Notes and Loans Payable [Member]
Cash paid during the year for interest:
Interest paid										40,537	27,164	27,283
Parent Company [Member] | Subordinated Debentures [Member]
Cash paid during the year for interest:
Interest paid										$ 14,573	$ 12,454	$ 11,833