Investments (Fair Value of Mortgage-Backed Securities by Credit Rating Origination) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Mortgage-backed securities	$ 2,857
2011 [Member]
Mortgage-backed securities	26
2010 [Member]
Mortgage-backed securities	3
2007 [Member]
Mortgage-backed securities	96
2006 [Member]
Mortgage-backed securities	177
2005 [Member]
Mortgage-backed securities	139
Pre - 2005 [Member]
Mortgage-backed securities	63
U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	2,353
AAA [Member]
Mortgage-backed securities	2,835
AAA [Member] | 2011 [Member]
Mortgage-backed securities	26
AAA [Member] | 2010 [Member]
Mortgage-backed securities	0
AAA [Member] | 2007 [Member]
Mortgage-backed securities	93
AAA [Member] | 2006 [Member]
Mortgage-backed securities	167
AAA [Member] | 2005 [Member]
Mortgage-backed securities	136
AAA [Member] | Pre - 2005 [Member]
Mortgage-backed securities	60
AAA [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	2,353
AA [Member]
Mortgage-backed securities	3
AA [Member] | 2011 [Member]
Mortgage-backed securities	0
AA [Member] | 2010 [Member]
Mortgage-backed securities	3
AA [Member] | 2007 [Member]
Mortgage-backed securities	0
AA [Member] | 2006 [Member]
Mortgage-backed securities	0
AA [Member] | 2005 [Member]
Mortgage-backed securities	0
AA [Member] | Pre - 2005 [Member]
Mortgage-backed securities	0
AA [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	0
A [Member]
Mortgage-backed securities	3
A [Member] | 2011 [Member]
Mortgage-backed securities	0
A [Member] | 2010 [Member]
Mortgage-backed securities	0
A [Member] | 2007 [Member]
Mortgage-backed securities	0
A [Member] | 2006 [Member]
Mortgage-backed securities	0
A [Member] | 2005 [Member]
Mortgage-backed securities	0
A [Member] | Pre - 2005 [Member]
Mortgage-backed securities	3
A [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	0
Non-Investment Grade [Member]
Mortgage-backed securities	16
Non-Investment Grade [Member] | 2011 [Member]
Mortgage-backed securities	0
Non-Investment Grade [Member] | 2010 [Member]
Mortgage-backed securities	0
Non-Investment Grade [Member] | 2007 [Member]
Mortgage-backed securities	3
Non-Investment Grade [Member] | 2006 [Member]
Mortgage-backed securities	10
Non-Investment Grade [Member] | 2005 [Member]
Mortgage-backed securities	3
Non-Investment Grade [Member] | Pre - 2005 [Member]
Mortgage-backed securities	0
Non-Investment Grade [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	$ 0