Employee Benefit Plans - Components of Pension and Other Retirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 28	$ 24	$ 29
Interest cost	45	47	43
Expected return on plan assets	(65)	(65)	(62)
Net amortization of prior service cost	(2)	(2)	(2)
Recognized actuarial loss	31	22	35
Net periodic benefit cost	$ 37	$ 26	$ 43
Assumptions used to determine benefit costs
Discount rate	4.60%	4.20%
Pension band increase (as a percent)	2.00%	2.00%	2.00%
Pension Plans [Member] | Weighted Average [Member]
Assumptions used to determine benefit costs
Discount rate	4.20%	5.00%	4.20%
Expected return on plan assets (as a percent)	7.80%	7.80%	7.80%
Rate of compensation increase (as a percent)	3.70%	3.70%	3.70%
Welfare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2	$ 3
Interest cost	13	15	14
Expected return on plan assets	(7)	(7)	(6)
Net amortization of prior service cost	(3)	(3)	(5)
Recognized actuarial loss	6	6	8
Net periodic benefit cost	$ 11	$ 13	$ 14
Assumptions used to determine benefit costs
Discount rate	4.40%	4.00%
Welfare Plans [Member] | Weighted Average [Member]
Assumptions used to determine benefit costs
Discount rate	4.00%	4.70%	4.00%
Expected return on plan assets (as a percent)	7.80%	7.80%	7.80%
Rate of compensation increase (as a percent)	3.70%	3.70%	3.80%