Investments and Cash (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Amortized Cost
Due within one year	$ 260
Due after one year through five years	1,477
Due after five years through 10 years	2,346
Due after 10 years	3,529
Estimated Fair Value
Due within one year	264
Due after one year through five years	1,539
Due after five years through 10 years	2,446
Due after 10 years	3,686
Estimated fair value	9,564		10,056
Fixed maturity securities held under trust for the benefit of reinsured companies	359		368
Eligible securities deposited for the benefit of policyholders	22		27
Percentage of the value of letter of credit of fixed maturity investments held in segregated account as collateral for letter of credit	120.00%
Fixed maturity investments in a segregated portfolio primarily to provide collateral for a LOC	3.5		3.5
Fair market value of company's pledged securities	680		660
Fair value of the securities purchased to mitigate insured losses	677		670
RMBS
Amortized Cost
Amortized cost	1,172
Estimated Fair Value
Estimated fair value	1,127
CMBS
Amortized Cost
Amortized cost	483
Estimated Fair Value
Estimated fair value	502
Fixed maturity securities
Amortized Cost
Amortized cost	9,267	[1]	9,346	[1]
Estimated Fair Value
Estimated fair value	$ 9,564

[1] Ratings in the tables above represent the lower of the Moody’s and S&P classifications except for bonds purchased for loss mitigation or risk management strategies, which use internal ratings classifications. The Company’s portfolio consists primarily of high-quality, liquid instruments.