Closed Block (Liabilities and Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Closed Block Liabilities
Future policy benefits	$ 40,180	$ 40,463
Other policy-related balances	204	222
Policyholder dividends payable	468	437
Policyholder dividend obligation	792	2,121	$ 1,931
Other liabilities	371	212
Total closed block liabilities	42,015	43,455
Assets Designated to the Closed Block
Fixed maturity securities available-for-sale, at estimated fair value	26,194	27,904
Equity securities, at estimated fair value	68	70
Mortgage loans	6,290	5,878
Policy loans	4,520	4,548
Real estate and real estate joint ventures	591	613
Other invested assets	694	731
Total investments	38,357	39,744
Accrued investment income	457	477
Premiums, reinsurance and other receivables; cash and cash equivalents designated to the closed block	70	14
Current income tax recoverable	36	35
Deferred income tax assets	19	36
Total assets designated to the closed block	38,939	40,306
Excess of closed block liabilities over assets designated to the closed block	3,076	3,149
Amounts included in AOCI:
Unrealized investment gains (losses), net of income tax	1,312	1,863
Unrealized gains (losses) on derivatives, net of income tax	16	(7)
Allocated to policyholder dividend obligation, net of income tax	(626)	(1,379)
Total amounts included in AOCI	702	477
Maximum future earnings to be recognized from closed block assets and liabilities	$ 3,778	$ 3,626