Pension and Other Postretirement Employee Benefit Plans - Plan Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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 (decrease) in PBO	$ 112	$ (533)
Additional years of historical data	2 years
Decrease in expense due to increased discount rates		$ 63
Defined benefit pension plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	6.50%
OBEP plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	5.75%
Assumed for next year	5.50%	6.00%
Ultimate rate	5.00%	5.00%	5.00%
Year when rate will reach ultimate	2017	2017	2017
Pension and other postretirement employee benefit plans, U.S.
Defined Benefit Plan Disclosure [Line Items]
Interest crediting rate	5.00%
Assumed rate of compensation increase	3.50%	3.50%	4.00%