Reinsurance (Income Statement Reinsurance Results) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Premiums:
Direct		$ 1,807,809	$ 1,720,896	$ 1,621,531
Assumed		230	194	359
Ceded		(1,757,231)	(1,666,384)	(2,447,832)
Net premiums		50,808	54,706	(825,942)
Policy charges and fee income:
Direct		3,248,574	3,459,134	2,804,446
Assumed		497,751	473,573	533,648
Ceded		(3,212,998)	(3,657,014)	(2,550,899)
Net policy charges and fee income	$ 129,155	533,327	275,693	787,195
Net investment income:
Direct		330,058	356,291	378,969
Assumed		1,581	1,484	1,411
Ceded		(6,352)	(5,365)	(4,430)
Net investment income		325,287	352,410	375,950
Asset administration fees:
Direct		346,727	340,461	310,178
Assumed		0	0	0
Ceded		(332,359)	(322,868)	(225,735)
Net asset administration fees		14,368	17,593	84,443
Other income:
Direct		68,931	62,830	50,475
Assumed		96	390	(161)
Ceded		(55)	(77)	21
Amortization of reinsurance income		3,992	4,606	(19,228)
Net other income		72,964	67,749	31,107
Realized investment gains (losses), net:
Direct		769,114	478,117	1,263,088
Assumed		0	0	0
Ceded		(936,392)	(558,303)	(504,639)
Realized investment gains (losses), net		(167,278)	(80,186)	758,449
Policyholders’ benefits (including change in reserves):
Direct		2,647,574	2,436,537	2,440,897
Assumed		599,589	584,909	596,196
Ceded		(3,097,664)	(3,039,862)	(3,312,658)
Net policyholders’ benefits (including change in reserves)	$ 51,736	149,499	(18,416)	(275,565)
Interest credited to policyholders’ account balances:
Direct		499,458	350,262	413,328
Assumed		141,307	135,123	131,953
Ceded		(468,772)	(316,994)	(244,061)
Net interest credited to policyholders’ account balances		171,993	168,391	301,220
Net reinsurance expense allowances, net of capitalization and amortization		(1,587,360)	(1,302,020)	(840,010)
Unaffiliated
Premiums:
Assumed		200	200	400
Ceded		(200)	0	0
Policy charges and fee income:
Ceded		(20,000)	(8,000)	(4,000)
Other income:
Assumed		100	400	(200)
Realized investment gains (losses), net:
Ceded		(34,000)	(20,000)	(30,000)
Policyholders’ benefits (including change in reserves):
Assumed		0	400	0
Ceded		$ (10,000)	$ 4,000	$ 5,000