Employee Benefit Plans - Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amounts recognized on the Consolidated Balance Sheets
Long-term asset	$ 78	$ 97
Pension Plans [Member]
Change in plan assets
Fair value of plan assets, beginning of period	914
Employee contributions	0	0
Benefits paid	(49)	(70)
Medicare Part D reimbursements	0	0
Fair value of plan assets, end of period	856	914
Change in benefit obligation
Benefit obligation, beginning of period	1,098	960
Service cost	28	24	$ 29
Interest cost	45	47	43
Actuarial loss (gain)	(55)	137
Medicare Part D reimbursements	0	0
Benefits paid	(49)	(70)
Employee contributions	0	0
Benefit obligation, end of period	1,067	1,098	960
Funded status at end of year	(220)	(192)
Amounts recognized on the Consolidated Balance Sheets
Long-term asset	78	97
Current liability	(4)	(2)
Long-term liability	(294)	(287)
Net liability at December 31,	(220)	(192)
Accumulated benefit obligation	$ 1,002	$ 1,027
Assumptions used to determine benefit obligations
Discount rate	4.60%	4.20%
Rate of compensation increase	3.70%	3.70%
Pension band increase	2.00%	2.00%
Welfare Plans [Member]
Change in plan assets
Fair value of plan assets, beginning of period	$ 90
Employee contributions	1	$ 1
Benefits paid	(20)	(19)
Medicare Part D reimbursements	1	1
Fair value of plan assets, end of period	90	90
Change in benefit obligation
Benefit obligation, beginning of period	334	326
Service cost	2	2	3
Interest cost	13	15	14
Actuarial loss (gain)	(13)	8
Medicare Part D reimbursements	1	1
Benefits paid	(20)	(19)
Employee contributions	1	1
Benefit obligation, end of period	318	334	326
Funded status at end of year	(219)	(235)
Amounts recognized on the Consolidated Balance Sheets
Long-term asset	0	0
Current liability	0	0
Long-term liability	(219)	(235)
Net liability at December 31,	$ (219)	$ (235)
Assumptions used to determine benefit obligations
Discount rate	4.40%	4.00%
Rate of compensation increase	3.70%	3.70%
Defined Benefit Plan Change In Fair Value Of Plan Assets [Member] | Pension Plans [Member]
Change in plan assets
Fair value of plan assets, beginning of period	$ 906	$ 907
Actual return on plan assets	(12)	68
Employee contributions	0	0
Employer contributions	2	1
Benefits paid	(49)	(70)
Medicare Part D reimbursements	0	0
Fair value of plan assets, end of period	847	906	907
Change in benefit obligation
Medicare Part D reimbursements	0	0
Benefits paid	(49)	(70)
Employee contributions	0	0
Defined Benefit Plan Change In Fair Value Of Plan Assets [Member] | Welfare Plans [Member]
Change in plan assets
Fair value of plan assets, beginning of period	99	93
Actual return on plan assets	1	5
Employee contributions	1	2
Employer contributions	17	17
Benefits paid	(20)	(19)
Medicare Part D reimbursements	1	1
Fair value of plan assets, end of period	99	99	$ 93
Change in benefit obligation
Medicare Part D reimbursements	1	1
Benefits paid	(20)	(19)
Employee contributions	$ 1	$ 2