Pension and Other Postretirement Benefits (Fair Value Measurements Level 3 Rollforward) (Details) - Pension benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value of plan assets, beginning of year	$ 3,734	$ 4,224
Fair value of plan assets, end of year	3,244	3,734
Mortgage-backed securities [Member] | Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value of plan assets, beginning of year	2	3
Contributions and distributions, net	0	(1)
Fair value of plan assets, end of year	$ 2	$ 2