Closed Block (Liabilities and Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Closed Block Liabilities
Future policy benefits	$ 40,106	$ 40,463
Other policy-related balances	287	222
Policyholder dividends payable	476	437
Policyholder dividend obligation	456	2,121	$ 1,931
Closed Block Liabilities, Income Tax Payable	2	0
Other liabilities	472	212
Total closed block liabilities	41,799	43,455
Assets Designated to the Closed Block
Fixed maturity securities available-for-sale, at estimated fair value	25,803	27,904
Equity securities, at estimated fair value	66	70
Mortgage loans	6,474	5,878
Policy loans	4,524	4,548
Real estate and real estate joint ventures	580	613
Other invested assets	686	731
Total investments	38,133	39,744
Accrued investment income	469	477
Premiums, reinsurance and other receivables; cash and cash equivalents designated to the closed block	92	14
Current income tax recoverable	52	35
Deferred income tax assets	0	36
Total assets designated to the closed block	38,746	40,306
Excess of closed block liabilities over assets designated to the closed block	3,053	3,149
Amounts included in AOCI:
Unrealized investment gains (losses), net of income tax	1,110	1,863
Unrealized gains (losses) on derivatives, net of income tax	17	(7)
Allocated to policyholder dividend obligation, net of income tax	(360)	(1,379)
Total amounts included in AOCI	767	477
Maximum future earnings to be recognized from closed block assets and liabilities	$ 3,820	$ 3,626