Closed Block (Liabilities and Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Closed Block Liabilities
Future policy benefits	$ 40,489	$ 40,834
Other policy-related balances	193	257
Policyholder dividends payable	483	443
Policyholder dividend obligation	2,201	1,931	$ 1,783
Current income tax payable	0	4
Other liabilities	277	196
Total closed block liabilities	43,643	43,665
Assets Designated to the Closed Block
Fixed maturity securities available-for-sale, at estimated fair value	27,541	27,220
Equity securities available-for-sale, at estimated fair value	71	100
Mortgage loans	5,904	5,935
Policy loans	4,542	4,553
Real estate and real estate joint ventures	628	655
Other invested assets	1,053	1,246
Total investments	39,739	39,709
Cash and cash equivalents	60	18
Accrued investment income	487	467
Premiums, reinsurance and other receivables	68	68
Assets Designated to Closed Block, Income Tax Receivable	30	0
Deferred income tax assets	109	177
Total assets designated to the closed block	40,493	40,439
Excess of closed block liabilities over assets designated to the closed block	3,150	3,226
Amounts included in accumulated other comprehensive income (loss):
Unrealized investment gains (losses), net of income tax	1,851	1,517
Unrealized gains (losses) on derivatives, net of income tax	23	95
Allocated to policyholder dividend obligation, net of income tax	(1,431)	(1,255)
Total amounts included in AOCI	443	357
Maximum future earnings to be recognized from closed block assets and liabilities	$ 3,593	$ 3,583