Financial Instruments (Offsetting of Financial Assets and Financial Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative Assets [Abstract]
Gross Amounts of Recognized Assets	$ 596	$ 628
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Presented in the Consolidated Balance Sheets	596	628
Financial Instruments	(11)	(13)
Cash Collateral Received	(337)	(189)
Non-Cash Collateral Received	(73)	(214)
Net Assets Exposed	175	212
Offsetting Securities Purchased under Agreements to Resell and Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets		1,590
Gross Amounts Offset in the Consolidated Balance Sheets		0
Net Presented in the Consolidated Balance Sheets		1,590
Financial Instruments		0
Cash Collateral Received		0
Non-Cash Collateral Received		(1,590)
Net Assets Exposed		0
Offsetting Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets		2,218
Gross Amounts Offset in the Consolidated Balance Sheets		0
Net Presented in the Consolidated Balance Sheets		2,218
Financial Instruments		(13)
Cash Collateral Received		(189)
Non-Cash Collateral Received		(1,804)
Net Assets Exposed		212
Offsetting Derivative Liabilities [Abstract]
Gross Amounts of Recognized Liabilities	13	16
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Presented in the Consolidated Balance Sheets	13	16
Financial Instruments	(11)	(13)
Cash Collateral Pledged	0	(3)
Non-Cash Collateral Pledged	0	0
Net Liabilities	$ 2	0
Offsetting Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities		2,428
Gross Amounts Offset in the Consolidated Balance Sheets		0
Net Presented in the Consolidated Balance Sheets		2,428
Financial Instruments		0
Cash Collateral Pledged		0
Non-Cash Collateral Pledged		(2,401)
Net Liabilities		27
Offsetting Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities		2,444
Gross Amounts Offset in the Consolidated Balance Sheets		0
Net Presented in the Consolidated Balance Sheets		2,444
Financial Instruments		(13)
Cash Collateral Pledged		(3)
Non-Cash Collateral Pledged		(2,401)
Net Liabilities		27
Cash and cash equivalents
Offsetting Securities Purchased under Agreements to Resell and Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets		1,140
Receivable under reverse repurchase agreements
Offsetting Securities Purchased under Agreements to Resell and Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets		$ 450