Employee Benefit Plans (Estimated Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 9,688	$ 9,009	$ 8,603
Pension Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,891	7,236
Pension Benefits | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,834	3,499
Pension Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,784	1,660
Pension Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,037	862
Pension Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	169	196
Pension Benefits | Municipals securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	335	313
Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	327	335
Pension Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	405	371
Pension Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	977	923
Pension Benefits | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	570	474
Pension Benefits | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	407	449
Pension Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	785	772
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35	78
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,282	2,674
Pension Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,433	1,774
Pension Benefits | Level 1 | Corporate fixed maturity securities [Member]
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	1,256	1,656
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	35	0
Pension Benefits | Level 1 | Municipals securities
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	135	118
Pension Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	0
Pension Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	797	854
Pension Benefits | Level 1 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	480	474
Pension Benefits | Level 1 | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	317	380
Pension Benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	30
Pension Benefits | Level 1 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33	16
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,770	5,624
Pension Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,447	5,453
Pension Benefits | Level 2 | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,833	3,499
Pension Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	528	4
Pension Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,037	862
Pension Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	134	196
Pension Benefits | Level 2 | Municipals securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	335	313
Pension Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	192	217
Pension Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	388	362
Pension Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	177	69
Pension Benefits | Level 2 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	90	0
Pension Benefits | Level 2 | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	69
Pension Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144	105
Pension Benefits | Level 2 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	(3)
Pension Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	636	711
Pension Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	9
Pension Benefits | Level 3 | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0	77
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	17
Pension Benefits | Level 3 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Municipals securities
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	10	9	7
Pension Benefits | Level 3 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Pension Benefits | Level 3 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0	0
Pension Benefits | Level 3 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	622	637	723
Pension Benefits | Level 3 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	65	76
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,434	1,386	$ 1,382
Other Postretirement Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,270	1,150
Other Postretirement Benefits | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	382	326
Other Postretirement Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	275	211
Other Postretirement Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102	79
Other Postretirement Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	27
Other Postretirement Benefits | Municipals securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	23
Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	398	429
Other Postretirement Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	55
Other Postretirement Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	154	235
Other Postretirement Benefits | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	80	113
Other Postretirement Benefits | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	74	122
Other Postretirement Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	0
Other Postretirement Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	452	479
Other Postretirement Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	243
Other Postretirement Benefits | Level 1 | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	20
Other Postretirement Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	269	210
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 securities
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	8	13
Other Postretirement Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	154	235
Other Postretirement Benefits | Level 1 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	80	113
Other Postretirement Benefits | Level 1 | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	74	122
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	1	1
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	982	907
Other Postretirement Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	973	907
Other Postretirement Benefits | Level 2 | Corporate fixed maturity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	362	306
Other Postretirement Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	1
Other Postretirement Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102	79
Other Postretirement Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	27
Other Postretirement Benefits | Level 2 | Municipals securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	23
Other Postretirement Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390	416
Other Postretirement Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	55
Other Postretirement Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2 | Common stock - foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	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	0	0
Other Postretirement Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Corporate fixed maturity securities [Member]
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 securities
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	0	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 | Common stock - domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Common stock - foreign
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