Derivative Instruments (Exposure from Credit Derivatives by NAIC Rating of the Underlying Credits) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Credit Derivatives [Line Items]
Notional	$ 320	$ 820
Fair value	1	3
Single Name [Member]
Credit Derivatives [Line Items]
Notional	320	820
Fair value	1	3
1 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	295	795
Fair value	1	3
2 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	25	25
Fair value	0	0
Investment Grade Subtotal [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	320	820
Fair value	1	3
3 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	0	0
Fair value	0	0
4 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	0	0
Fair value	0	0
5 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	0	0
Fair value	0	0
6 [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	0	0
Fair value	0	0
Below Investment Grade Subtotal [Member] | Single Name [Member]
Credit Derivatives [Line Items]
Notional	0	0
Fair value	$ 0	$ 0