Income Tax (Narrative) (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Nov. 19, 2015	Sep. 18, 2015	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal corporate income tax rate				35.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount						$ (41)
Foreign tax rate differential				$ (95)	$ (332)	(555)
Deferred Federal Income Tax Expense (Benefit)				(2,373)	(827)	194
Income Tax Assessment, Income Taxes and Interest Paid		$ 444
Other expenses				13,621	13,749	14,777
Income Tax Reconciliation, Deductions, Dividends, Related To Separate Accounts				61	63	66
Subsequent Event [Member]
Federal corporate income tax rate			21.00%
Impact of U.S. Tax Reform
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount				(1,519)	$ 0	0
Deferred Federal Income Tax Expense (Benefit)				(1,790)
Foreign Tax Authority
Foreign tax rate differential				350
Foreign Tax Authority | Impact of U.S. Tax Reform
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount				170
Foreign dividends [Member] | Foreign Tax Authority
Foreign tax rate differential				180
Compensation and fringe benefits [Member] | Impact of U.S. Tax Reform
Deferred Federal Income Tax Expense (Benefit)				8
Alternative Minimum Tax Credits [Member] | Impact of U.S. Tax Reform
Deferred Federal Income Tax Expense (Benefit)				$ 9
Other Assets [Member]
Transition tax rate				8.00%
Cash and Cash Equivalents
Transition tax rate				15.50%
Alternative Minimum Tax Credits [Member]
Transition tax rate				6.60%
Income Tax Refund [Member]
Other expenses	$ 9
Maximum
True Up Of Prior Year Tax Return Included In Current Year Benefit Associated With Dividend Received Deduction Related to Separate Accounts				$ 1		$ 1
Certain State and Foreign Net Operating Loss Carryforwards [Member]
Valuation allowance, change during year				8
Foreign Currency Movement [Member]
Valuation allowance, change during year				19
Balance Sheet Reclassification With Other Deferred Tax Assets [Member]
Valuation allowance, change during year				$ 17