Reinsurance (Income Statement Reinsurance Results) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums:
Direct	$ 33,908	$ 39,326	$ 33,250
Assumed	32,890	860,831	0
Ceded	(3,225)	(3,318)	(23,463)
Net premiums	63,573	896,839	9,787
Policy charges and fee income:
Direct	622,099	647,226	743,956
Assumed	1,632,132	1,153,752	0
Ceded	(44,652)	(45,754)	(3,133)
Net policy charges and fee income	2,209,579	1,755,224	740,823
Asset administration fees and other income:
Direct	129,847	103,892	177,479
Assumed	293,275	205,221	0
Ceded	(9,747)	(9,729)	0
Net asset administration fees and other income	413,375	299,384	177,479
Realized investment gains (losses), net:
Direct	(1,335,253)	(3,612,578)	247,525
Assumed	554,686	(81,510)	0
Ceded	(24,833)	251,328	(241,473)
Realized investment gains (losses), net	(805,400)	(3,442,760)	6,052
Policyholders' benefits (including change in reserves):
Direct	52,477	74,438	81,719
Assumed	46,375	553,280	0
Ceded	15,216	(23,661)	(21,258)
Net policyholders' benefits (including change in reserves)	114,068	604,057	60,461
Interest credited to policyholders’ account balances:
Direct	9,834	74,389	225,555
Assumed	24,708	(1,551)	0
Ceded	(4,262)	(3,949)	0
Net interest credited to policyholders’ account balances	30,280	68,889	225,555
Reinsurance expense allowances and general and administrative expenses, net of capitalization and amortization	725,749	563,027	(6,054)
Unaffiliated activity
Policy charges and fee income:
Ceded	(2,000)	(2,000)	(3,000)
Policyholders' benefits (including change in reserves):
Ceded	$ (100)	$ (300)	$ (100)