Financial Instruments (Securities Lending Program) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Total	$ 3,266	$ 2,778
Securities lending payable	3,266	2,778
Amounts related to agreements not included in securities lending in offsetting disclosure	0	$ 0
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total	2,164
Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total	1,001
30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	101
U.S. government notes
Assets Sold under Agreements to Repurchase [Line Items]
Total	2,881
U.S. government notes | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total	1,779
U.S. government notes | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total	1,001
U.S. government notes | 30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
U.S. government notes | Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	101
U.S. government agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total	192
U.S. government agencies | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total	192
U.S. government agencies | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
U.S. government agencies | 30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
U.S. government agencies | Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
Corporate debt securities
Assets Sold under Agreements to Repurchase [Line Items]
Total	193
Corporate debt securities | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total	193
Corporate debt securities | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
Corporate debt securities | 30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	0
Corporate debt securities | Greater Than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Total	$ 0