Employee Pension and Other Postretirement Benefits - Executive Supplemental Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan and Other Postretirement Benefits Plan
Employee pre-tax covered compensation percent	75.00%
Employee after-tax covered compensation percent	10.00%
Combined maximum employee contribution percent	75.00%
Employee pre-tax gross pay for which the employer contributes a match (as a percent)	4.00%
Contribution plan expenses	$ 47	$ 37	$ 32
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Accrued liability for ESBPs	$ 101	$ 98
Matching Range 1
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match (as a percent)	100.00%
Employee pre-tax gross pay for which the employer contributes a match (as a percent)	3.00%
Matching Range 2
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match (as a percent)	50.00%
Employee pre-tax gross pay for which the employer contributes a match (as a percent)	2.00%