Schedule II - Condensed Financial Information of Registrant (Condensed Statements of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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
Revenues:
Net investment income	$ 512,709	$ 500,202	$ 493,489	$ 485,597	$ 475,633	$ 463,583	$ 459,830	$ 450,826	$ 1,991,997	$ 1,849,872	$ 1,692,192
Change in fair value of derivatives	661,993	362,525	266,820	386,533	95,391	103,794	39,099	(74,065)	1,677,871	164,219	(336,146)
Loss on extinguishment of debt	0	(18,389)	(428)	0					(18,817)	0	0
Total revenues	1,239,270	905,953	819,128	927,301	622,816	630,060	549,802	417,604	3,891,652	2,220,282	1,518,937
Expenses:
Change in fair value of embedded derivatives									919,735	543,465	(464,698)
Interest expense on notes and loan payable									30,368	28,248	28,849
Interest expense on subordinated debentures issued to subsidiary trusts									14,124	12,958	12,239
Other operating costs and expenses									111,691	102,231	96,218
Total benefits and expenses									3,575,381	2,090,035	1,181,623
Income tax expense									141,626	47,004	117,484
Net income	$ 36,803	$ 56,957	$ 26,946	$ 53,939	$ 120,796	$ (7,420)	$ 14,708	$ (44,841)	174,645	83,243	219,830
Parent Company [Member]
Revenues:
Net investment income									492	78	62
Dividends from subsidiary trusts									410	384	363
Investment advisory fees									83,941	75,706	65,957
Surplus note interest from subsidiary									4,080	4,080	4,080
Change in fair value of derivatives									(412)	(810)	(8,225)
Loss on extinguishment of debt									(18,817)	0	0
Total revenues									69,694	79,438	62,237
Expenses:
Change in fair value of embedded derivatives									0	0	(4,516)
Interest expense on notes and loan payable									30,368	28,248	28,849
Interest expense on subordinated debentures issued to subsidiary trusts									14,124	12,958	12,239
Other operating costs and expenses									9,234	8,551	8,195
Total benefits and expenses									53,726	49,757	44,767
Income before income taxes and equity in undistributed income of subsidiaries									15,968	29,681	17,470
Income tax expense									6,895	12,073	7,338
Income before equity in undistributed income of subsidiaries									9,073	17,608	10,132
Equity in undistributed income of subsidiaries									165,572	65,635	209,698
Net income									$ 174,645	$ 83,243	$ 219,830