Income Tax (Net Operating and Capital Loss Carryforwards for Tax Purposes) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Capital loss carryforwards	$ 1,322	$ 1,375
Domestic Tax Authority
Net Operating Loss Carryforwards	2,104
Domestic Tax Authority | Capital Loss Carryforwards
Capital loss carryforwards	27
Domestic Tax Authority | 2018-2022
Net Operating Loss Carryforwards	1
Domestic Tax Authority | 2018-2022 | Capital Loss Carryforwards
Capital loss carryforwards	27
Domestic Tax Authority | 2023-2027
Net Operating Loss Carryforwards	0
Domestic Tax Authority | 2023-2027 | Capital Loss Carryforwards
Capital loss carryforwards	0
Domestic Tax Authority | 2028-2032
Net Operating Loss Carryforwards	8
Domestic Tax Authority | 2028-2032 | Capital Loss Carryforwards
Capital loss carryforwards	0
Domestic Tax Authority | 2033-2037
Net Operating Loss Carryforwards	2,095
Domestic Tax Authority | 2033-2037 | Capital Loss Carryforwards
Capital loss carryforwards	0
Domestic Tax Authority | Indefinite
Net Operating Loss Carryforwards	0
Domestic Tax Authority | Indefinite | Capital Loss Carryforwards
Capital loss carryforwards	0
State and Local Jurisdiction
Net Operating Loss Carryforwards	128
State and Local Jurisdiction | 2018-2022
Net Operating Loss Carryforwards	49
State and Local Jurisdiction | 2023-2027
Net Operating Loss Carryforwards	64
State and Local Jurisdiction | 2028-2032
Net Operating Loss Carryforwards	13
State and Local Jurisdiction | 2033-2037
Net Operating Loss Carryforwards	2
State and Local Jurisdiction | Indefinite
Net Operating Loss Carryforwards	0
Foreign Tax Authority
Net Operating Loss Carryforwards	471
Capital loss carryforwards	264
Foreign Tax Authority | 2018-2022
Net Operating Loss Carryforwards	46
Capital loss carryforwards	42
Foreign Tax Authority | 2023-2027
Net Operating Loss Carryforwards	28
Capital loss carryforwards	200
Foreign Tax Authority | 2028-2032
Net Operating Loss Carryforwards	0
Capital loss carryforwards	1
Foreign Tax Authority | 2033-2037
Net Operating Loss Carryforwards	0
Capital loss carryforwards	0
Foreign Tax Authority | Indefinite
Net Operating Loss Carryforwards	397
Capital loss carryforwards	$ 21