Financial Instruments (Securities Lending Program) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Total		$ 2,428
Gross amount of recognized liabilities for securities lending in offsetting disclosure	$ 0	2,428
Amounts related to agreements not included in securities lending in offsetting disclosure		0
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total		2,014
Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total		108
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		306
U.S. government notes
Assets Sold under Agreements to Repurchase [Line Items]
Total		1,659
U.S. government notes | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total		1,322
U.S. government notes | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total		31
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		306
U.S. government agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total		581
U.S. government agencies | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total		504
U.S. government agencies | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Total		77
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		188
Corporate debt securities | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Total		188
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