Closed Block (Liabilities and Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Closed Block Liabilities
Future policy benefits	$ 40,285	$ 40,463
Other policy-related balances	213	222
Policyholder dividends payable	462	437
Policyholder dividend obligation	1,277	2,121	$ 1,931
Other liabilities	285	212
Total closed block liabilities	42,522	43,455
Assets Designated to the Closed Block
Fixed maturity securities available-for-sale, at estimated fair value	26,815	27,904
Equity securities, at estimated fair value	68	70
Mortgage loans	6,040	5,878
Policy loans	4,532	4,548
Real estate and real estate joint ventures	595	613
Other invested assets	607	731
Total investments	38,657	39,744
Accrued investment income	475	477
Premiums, reinsurance and other receivables; cash and cash equivalents designated to the closed block	208	14
Current income tax recoverable	40	35
Deferred income tax assets	23	36
Total assets designated to the closed block	39,403	40,306
Excess of closed block liabilities over assets designated to the closed block	3,119	3,149
Amounts included in AOCI:
Unrealized investment gains (losses), net of income tax	1,711	1,863
Unrealized gains (losses) on derivatives, net of income tax	(57)	(7)
Allocated to policyholder dividend obligation, net of income tax	(1,009)	(1,379)
Total amounts included in AOCI	645	477
Maximum future earnings to be recognized from closed block assets and liabilities	$ 3,764	$ 3,626