Fair Value (Financial Instruments Carried at Other Than Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Policy loans	$ 9,669	$ 9,511
Liabilities
Collateral financing arrangement	1,121	1,274
Junior subordinated debt securities	3,144	3,169
Separate account liabilities	205,001	195,578
Carrying Value
Assets
Mortgage loans	68,211	64,601
Policy loans	9,669	9,511
Other limited partnership interests	219	340
Other invested assets	443	497
Premiums, reinsurance and other receivables	4,155	4,088
Other assets	285	237
Liabilities
Policyholder account balances	114,355	108,255
Long-term debt	15,675	16,422
Collateral financing arrangement	1,121	1,274
Junior subordinated debt securities	3,144	3,169
Other liabilities	3,208	1,767
Separate account liabilities	124,011	118,385
Estimated Fair Value
Assets
Mortgage loans	69,797	65,742
Policy loans	11,512	11,256
Other limited partnership interests	216	371
Other invested assets	443	497
Premiums, reinsurance and other receivables	4,339	4,279
Other assets	328	269
Liabilities
Policyholder account balances	116,534	110,359
Long-term debt	17,773	17,972
Collateral financing arrangement	894	978
Junior subordinated debt securities	4,319	3,982
Other liabilities	3,841	1,768
Separate account liabilities	124,011	118,385
Estimated Fair Value | Level 1
Assets
Mortgage loans	0	0
Policy loans	0	0
Other limited partnership interests	0	0
Other invested assets	0	145
Premiums, reinsurance and other receivables	0	0
Other assets	0	0
Liabilities
Policyholder account balances	0	0
Long-term debt	0	0
Collateral financing arrangement	0	0
Junior subordinated debt securities	0	0
Other liabilities	0	0
Separate account liabilities	0	0
Estimated Fair Value | Level 2
Assets
Mortgage loans	0	0
Policy loans	336	335
Other limited partnership interests	0	0
Other invested assets	0	0
Premiums, reinsurance and other receivables	1,283	1,152
Other assets	189	198
Liabilities
Policyholder account balances	0	0
Long-term debt	17,773	17,972
Collateral financing arrangement	0	0
Junior subordinated debt securities	4,319	3,982
Other liabilities	1,496	1,493
Separate account liabilities	124,011	118,385
Estimated Fair Value | Level 3
Assets
Mortgage loans	69,797	65,742
Policy loans	11,176	10,921
Other limited partnership interests	216	371
Other invested assets	443	352
Premiums, reinsurance and other receivables	3,056	3,127
Other assets	139	71
Liabilities
Policyholder account balances	116,534	110,359
Long-term debt	0	0
Collateral financing arrangement	894	978
Junior subordinated debt securities	0	0
Other liabilities	2,345	275
Separate account liabilities	$ 0	$ 0