Consolidated Statements of Operations - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Premiums and other considerations		$ 34,228	$ 43,767	$ 36,048
Annuity product charges		200,494	173,579	136,168
Net investment income		1,991,997	1,849,872	1,692,192
Change in fair value of derivatives		1,677,871	164,219	(336,146)
Net realized gains on investments, excluding other than temporary impairment (OTTI) losses		10,509	11,524	10,211
OTTI losses on investments:
Total OTTI losses		(2,758)	(21,349)	(25,547)
Portion of OTTI losses recognized in (from) other comprehensive income		(1,872)	(1,330)	6,011
Net OTTI losses recognized in operations		(4,630)	(22,679)	(19,536)
Loss on extinguishment of debt		(18,817)	0	0
Total revenues		3,891,652	2,220,282	1,518,937
Benefits and expenses:
Insurance policy benefits and change in future policy benefits		43,219	52,483	45,458
Interest sensitive and index product benefits		2,023,668	725,472	968,053
Amortization of deferred sales inducements		176,612	251,166	209,390
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		14,124	12,958	12,239
Amortization of deferred policy acquisition costs		255,964	374,012	286,114
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 before income taxes		316,271	130,247	337,314
Income tax expense		141,626	47,004	117,484
Net income		$ 174,645	$ 83,243	$ 219,830
Earnings per common share		$ 1.96	$ 0.98	$ 2.78
Earnings per common share - assuming dilution		$ 1.93	$ 0.97	$ 2.72
Weighted average common shares outstanding: earnings per common share	[1]	88,982,442	84,793,151	78,936,828
Weighted average common shares outstanding: earnings per common share - assuming dilution		90,311,008	85,605,169	80,960,568

[1] Weighted average common shares outstanding include shares vested under the NMO Deferred Compensation Plan.