Fair Value (Financial Instruments Carried at Other Than Fair Value) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets
Policy loans	$ 9,744	$ 9,669
Liabilities
Collateral financing arrangement	1,108	1,121
Junior subordinated debt securities	3,145	3,144
Separate account liabilities	196,358	205,001
Carrying Value
Assets
Mortgage loans	70,617	68,211
Policy loans	9,744	9,669
Other limited partnership interests		219
Other invested assets	1,264	443
Premiums, reinsurance and other receivables	4,358	4,155
Other assets	345	285
Liabilities
Policyholder account balances	114,715	114,355
Long-term debt	15,696	15,675
Collateral financing arrangement	1,108	1,121
Junior subordinated debt securities	3,145	3,144
Other liabilities	3,810	3,208
Separate account liabilities	118,151	124,011
Estimated Fair Value
Assets
Mortgage loans	71,845	69,797
Policy loans	11,423	11,512
Other limited partnership interests		216
Other invested assets	1,264	443
Premiums, reinsurance and other receivables	4,475	4,339
Other assets	374	328
Liabilities
Policyholder account balances	116,193	116,534
Long-term debt	16,973	17,773
Collateral financing arrangement	898	894
Junior subordinated debt securities	4,073	4,319
Other liabilities	4,365	3,841
Separate account liabilities	118,151	124,011
Estimated Fair Value | Level 1
Assets
Mortgage loans	0	0
Policy loans	0	0
Other limited partnership interests		0
Other invested assets	0	0
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	341	336
Other limited partnership interests		0
Other invested assets	812	0
Premiums, reinsurance and other receivables	1,450	1,283
Other assets	175	189
Liabilities
Policyholder account balances	0	0
Long-term debt	16,973	17,773
Collateral financing arrangement	0	0
Junior subordinated debt securities	4,073	4,319
Other liabilities	2,113	1,496
Separate account liabilities	118,151	124,011
Estimated Fair Value | Level 3
Assets
Mortgage loans	71,845	69,797
Policy loans	11,082	11,176
Other limited partnership interests		216
Other invested assets	452	443
Premiums, reinsurance and other receivables	3,025	3,056
Other assets	199	139
Liabilities
Policyholder account balances	116,193	116,534
Long-term debt	0	0
Collateral financing arrangement	898	894
Junior subordinated debt securities	0	0
Other liabilities	2,252	2,345
Separate account liabilities	$ 0	$ 0