Change in Accounting Principle (Narrative) (Details) - Amortization Of Actuarial Gains And Losses For U.S. Government Defined-benefit Pension Plan Participants $ in Millions	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in AOL for retrospective application of change in accounting principle	$ 366
Decrease tax asset for retrospective application of change in accounting principle	40
Increase in deferred tax liability for retrospective application of change in accounting principle	57
Increase in other contract costs for retrospective application of change in accounting principle	$ 463