Consolidated Statements of Cash Flows (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Statement of Cash Flows [Abstract]
Net cash flows provided by (used in) operating activities	$ 146	$ (223)
Fixed maturity securities:
Purchases	(1,563)	(1,322)
Sales	812	683
Maturities	643	758
Net sales (purchases) of short-term investments	44	282
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	553	407
Acquisition of Municipal Assurance Corp., net of cash acquired	0	(91)
Other	81	85
Net cash flows provided by (used in) investing activities	570	802
Financing activities
Proceeds from issuance of common stock	0	173
Dividends paid	(57)	(51)
Repurchases of common stock	(259)	(24)
Share activity under option and incentive plans	0	(3)
Net paydowns of financial guaranty variable interest entities’ liabilities	(409)	(553)
Repayment of long-term debt	(22)	(204)
Net cash flows provided by (used in) financing activities	(747)	(662)
Effect of exchange rate changes	(1)	1
Increase (decrease) in cash	(32)	(82)
Cash at beginning of period	138	215
Cash at end of period	106	133
Cash paid (received) during the period for:
Income taxes	81	(11)
Interest	$ 47	$ 56