Pension and Other Postretirement Employee Benefit Plans - Plan Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Investment advisor definition of long-term	10 years
Forward rate implied by the Citigroup pension discount curve at which excess cash is assumed to be reinvested	1 year
Increase in expense due to decreased discount rates	$ 45
Defined benefit pension plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	6.00%
Rate of compensation increase	2.30%	3.50%
OBEP plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	5.00%
Assumed for next year	5.00%	5.50%
Ultimate rate	5.00%	5.00%	5.00%
Pension and other postretirement employee benefit plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Interest crediting rate	5.00%