Quarterly Financial Information (Unaudited) (Unaudited Quarterly Results of Operations) (Details) - USD ($) $ / shares in Units, $ 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
Premiums and product charges	$ 64,925	$ 60,500	$ 56,323	$ 52,974	$ 60,508	$ 60,406	$ 52,582	$ 43,850
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)
Net realized gains on investments, excluding other than temporary impairment (OTTI) losses	2,719	1,579	3,873	2,338	844	5,256	2,737	2,687	10,509	11,524	10,211
Net OTTI losses recognized in operations	(3,076)	(464)	(949)	(141)	(9,560)	(2,979)	(4,446)	(5,694)	(4,630)	(22,679)	(19,536)
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
Net income	$ 36,803	$ 56,957	$ 26,946	$ 53,939	$ 120,796	$ (7,420)	$ 14,708	$ (44,841)	$ 174,645	$ 83,243	$ 219,830
Earnings (loss) per common share	$ 0.41	$ 0.64	$ 0.30	$ 0.61	$ 1.37	$ (0.09)	$ 0.18	$ (0.55)	$ 1.96	$ 0.98	$ 2.78
Earnings (loss) per common share - assuming dilution	$ 0.41	$ 0.63	$ 0.30	$ 0.60	$ 1.35	$ (0.09)	$ 0.18	$ (0.55)	$ 1.93	$ 0.97	$ 2.72