Closed Block (Liabilities and Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Closed Block Liabilities
Future policy benefits	$ 39,727	$ 40,032
Other policy-related balances	422	317
Policyholder dividends payable	453	431
Policyholder dividend obligation	1,116	428	$ 2,121
Deferred income tax liability	36	28
Other liabilities	200	328
Total closed block liabilities	41,954	41,564
Assets Designated to the Closed Block
Fixed maturity securities available-for-sale, at estimated fair value	25,616	25,354
Equity securities, at estimated fair value	63	61
Contractholder-directed equity securities and fair value option securities, at estimated fair value	47	43
Mortgage loans	6,994	6,778
Policy loans	4,505	4,527
Real estate and real estate joint ventures	556	544
Other invested assets	592	643
Total investments	38,373	37,950
Accrued investment income	451	443
Premiums, reinsurance and other receivables	70	83
Current income tax recoverable	71	69
Total assets designated to the closed block	38,965	38,545
Excess of closed block liabilities over assets designated to the closed block	2,989	3,019
Amounts included in AOCI:
Unrealized investment gains (losses), net of income tax	1,675	1,089
Unrealized gains (losses) on derivatives, net of income tax	74	86
Allocated to policyholder dividend obligation, net of income tax	(882)	(338)
Total amounts included in AOCI	867	837
Maximum future earnings to be recognized from closed block assets and liabilities	$ 3,856	$ 3,856