Employee Benefit Plans (Pension plan and other postretirement plan assets that are measured at fair value and included in the Level 3 fair value category) (Details) (Level 3, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	$ 1,285	$ 1,278	$ 1,317
Actual return on plan assets:
Relating to assets still held at the reporting date	105	128	118
Relating to assets sold during the period	(62)	(67)
Purchases	129	215	182
Sales	(267)	(269)	(339)
Fair value of plan assets at end of year	1,190	1,285	1,278
Pension Benefits | Real estate:
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	304	271	344
Actual return on plan assets:
Relating to assets still held at the reporting date	21	38	8
Relating to assets sold during the period	(8)	(8)
Purchases	35	57	56
Sales	(31)	(54)	(137)
Fair value of plan assets at end of year	321	304	271
Pension Benefits | Private equity
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	448	400	344
Actual return on plan assets:
Relating to assets still held at the reporting date	46	70	56
Relating to assets sold during the period	(41)	(34)
Purchases	79	76	90
Sales	(76)	(64)	(90)
Fair value of plan assets at end of year	456	448	400
Pension Benefits | Debt
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	243	262	241
Actual return on plan assets:
Relating to assets still held at the reporting date	17	10	27
Relating to assets sold during the period	(11)	(10)
Purchases	15	34	36
Sales	(72)	(53)	(42)
Fair value of plan assets at end of year	192	243	262
Pension Benefits | Hedge funds
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	290	345	388
Actual return on plan assets:
Relating to assets still held at the reporting date	21	10	27
Relating to assets sold during the period	(2)	(15)
Purchases	0	48	0
Sales	(88)	(98)	(70)
Fair value of plan assets at end of year	221	290	345
Other Postretirement Plans
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	137	140	135
Actual return on plan assets:
Relating to assets still held at the reporting date	9	15	12
Relating to assets sold during the period	(1)
Purchases	9	16	20
Sales	(30)	(28)	(27)
Fair value of plan assets at end of year	124	137	140
Other Postretirement Plans | Real estate:
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	24	22	26
Actual return on plan assets:
Relating to assets still held at the reporting date	1	3	0
Relating to assets sold during the period	0	0
Purchases	2	3	3
Sales	(3)	(4)	(7)
Fair value of plan assets at end of year	24	24	22
Other Postretirement Plans | Private equity
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	63	61	54
Actual return on plan assets:
Relating to assets still held at the reporting date	3	11	9
Relating to assets sold during the period	(1)	(4)
Purchases	6	8	9
Sales	(13)	(13)	(11)
Fair value of plan assets at end of year	58	63	61
Other Postretirement Plans | Debt
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	36	40	36
Actual return on plan assets:
Relating to assets still held at the reporting date	4	1	2
Relating to assets sold during the period	0	(1)
Purchases	1	3	8
Sales	(10)	(7)	(6)
Fair value of plan assets at end of year	31	36	40
Other Postretirement Plans | Hedge funds
Changes in fair value of plan assets:
Fair value of plan assets at beginning of year	14	17	19
Actual return on plan assets:
Relating to assets still held at the reporting date	1	0	1
Relating to assets sold during the period	0	(1)
Purchases	0	2	0
Sales	(4)	(4)	(3)
Fair value of plan assets at end of year	$ 11	$ 14	$ 17