Quarterly Financial Information (Unaudited) (Unaudited Quarterly Results of Operations) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums and product charges	$ 71,568	$ 68,799	$ 64,826	$ 58,376	$ 64,824	$ 65,605	$ 60,763	$ 59,776
Net investment income	588,217	590,412	570,568	558,438	554,355	549,391	533,282	510,784	$ 2,307,635	$ 2,147,812	$ 1,991,997
Change in fair value of derivatives	466,434	(20,042)	76,045	384,469	(1,054,281)	595,311	132,205	(451,083)	906,906	(777,848)	1,677,871
Net realized gains (losses) on investments, excluding OTTI losses	7,029	4,328	(3,832)	(563)	3,097	(2,196)	(38,381)	302	6,962	(37,178)	10,509
Net OTTI losses recognized in operations	(17,412)	(101)	(1,213)	0	(18,980)	(14,373)	(2,396)	(907)	(18,726)	(36,656)	(4,630)
Loss on extinguishment of debt	(2,001)	0	0	0					(2,001)	0	(18,817)
Total revenues	1,113,835	643,396	706,394	1,000,720	(450,985)	1,193,738	685,473	118,872	3,464,345	1,547,098	3,891,652
Net income	$ 220,150	$ 37,360	$ 18,590	$ (30,010)	$ 53,823	$ 169,328	$ 93,903	$ 140,962	$ 246,090	$ 458,016	$ 174,645
Earnings (loss) per common share	$ 2.41	$ 0.41	$ 0.20	$ (0.33)	$ 0.59	$ 1.87	$ 1.04	$ 1.57	$ 2.70	$ 5.07	$ 1.96
Earnings (loss) per common share - assuming dilution	$ 2.40	$ 0.41	$ 0.20	$ (0.33)	$ 0.59	$ 1.85	$ 1.03	$ 1.55	$ 2.68	$ 5.01	$ 1.93