General Adoption of New Accounting Pronouncements (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ 20	$ 470	$ 450
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, Tax	$ 7
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(417)			$ (417)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	$ (454)
Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset					$ 800