Investments (Fair Value of Mortgage-Backed Securities by Credit Rating Origination) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Mortgage-backed securities	$ 3,065
2014
Mortgage-backed securities	100
2013
Mortgage-backed securities	157
2012
Mortgage-backed securities	109
2011
Mortgage-backed securities	18
2010
Mortgage-backed securities	25
2009
Mortgage-backed securities	7
Pre 2009
Mortgage-backed securities	410
U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	2,239
AAA [Member]
Mortgage-backed securities	3,048
AAA [Member] | 2014
Mortgage-backed securities	100
AAA [Member] | 2013
Mortgage-backed securities	157
AAA [Member] | 2012
Mortgage-backed securities	109
AAA [Member] | 2011
Mortgage-backed securities	18
AAA [Member] | 2010
Mortgage-backed securities	25
AAA [Member] | 2009
Mortgage-backed securities	7
AAA [Member] | Pre 2009
Mortgage-backed securities	395
AAA [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	2,237
AA [Member]
Mortgage-backed securities	4
AA [Member] | 2014
Mortgage-backed securities	0
AA [Member] | 2013
Mortgage-backed securities	0
AA [Member] | 2012
Mortgage-backed securities	0
AA [Member] | 2011
Mortgage-backed securities	0
AA [Member] | 2010
Mortgage-backed securities	0
AA [Member] | 2009
Mortgage-backed securities	0
AA [Member] | Pre 2009
Mortgage-backed securities	2
AA [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	2
External Credit Rating, Non Investment Grade [Member]
Mortgage-backed securities	13
External Credit Rating, Non Investment Grade [Member] | 2014
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | 2013
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | 2012
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | 2011
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | 2010
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | 2009
Mortgage-backed securities	0
External Credit Rating, Non Investment Grade [Member] | Pre 2009
Mortgage-backed securities	13
External Credit Rating, Non Investment Grade [Member] | U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities	$ 0