Employee Benefit Plans (Estimated Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,230	$ 8,948	$ 9,688
Pension Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,328	7,301
Pension Benefits | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,750	3,351
Pension Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,056	1,785
Pension Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	996	837
Pension Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	88
Pension Benefits | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	280	240
Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	192	199
Pension Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	948	801
Pension Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,177	901
Pension Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	712	708
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	38
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,922	2,284
Pension Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,927	1,525
Pension Benefits | Level 1 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,599	1,314
Pension Benefits | Level 1 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Pension Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	328	210
Pension Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	962	706
Pension Benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	20
Pension Benefits | Level 1 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	33
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,622	5,973
Pension Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,401	5,774
Pension Benefits | Level 2 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,750	3,350
Pension Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	457	471
Pension Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	996	837
Pension Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	88
Pension Benefits | Level 2 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	280	240
Pension Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	192	198
Pension Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	620	590
Pension Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	215	195
Pension Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Pension Benefits | Level 2 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Pension Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	686	691
Pension Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
Pension Benefits | Level 3 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1	1
Pension Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1	10
Pension Benefits | Level 3 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0	3
Pension Benefits | Level 3 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	686	688	622
Pension Benefits | Level 3 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1	0
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,468	1,360	$ 1,434
Other Postretirement Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,409	1,186
Other Postretirement Benefits | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	278	313
Other Postretirement Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	259	268
Other Postretirement Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	90
Other Postretirement Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	16
Other Postretirement Benefits | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	29
Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	407	398
Other Postretirement Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	373	72
Other Postretirement Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	173
Other Postretirement Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	493	428
Other Postretirement Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	434	272
Other Postretirement Benefits | Level 1 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	259	268
Other Postretirement Benefits | Level 1 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	1
Other Postretirement Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151	3
Other Postretirement Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	155
Other Postretirement Benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	972	932
Other Postretirement Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	972	914
Other Postretirement Benefits | Level 2 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	278	313
Other Postretirement Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	90
Other Postretirement Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	16
Other Postretirement Benefits | Level 2 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	29
Other Postretirement Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	383	397
Other Postretirement Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	219	69
Other Postretirement Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	18
Other Postretirement Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Other Postretirement Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Other Postretirement Benefits | Level 3 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Other Postretirement Benefits | Level 3 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0