Subsidiary Information (Details 3) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Condensed financial statements
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)
Intercompany debt	0	0
Investment in subsidiary	0	0
Other	81	85
Net cash flows provided by (used in) investing activities	570	802
Cash flows from financing activities
Proceeds from issuance of common stock	0	173
Return of capital	0	0
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)
Intercompany debt	0	0
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
Assured Guaranty Ltd. (Parent)
Condensed financial statements
Net cash flows provided by (used in) operating activities	108	98
Fixed maturity securities:
Purchases	0	(160)
Sales	171	0
Maturities	21	1
Net sales (purchases) of short-term investments	16	(34)
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	0	0
Acquisition of Municipal Assurance Corp., net of cash acquired		0
Intercompany debt	0	0
Investment in subsidiary	0	0
Other	0	0
Net cash flows provided by (used in) investing activities	208	(193)
Cash flows from financing activities
Proceeds from issuance of common stock		173
Return of capital	0	0
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	0	0
Repayment of long-term debt	0	0
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(316)	95
Effect of exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash at beginning of period	0	0
Cash at end of period	0	0
AGUS (Issuer)
Condensed financial statements
Net cash flows provided by (used in) operating activities	123	23
Fixed maturity securities:
Purchases	(35)	(1)
Sales	1	0
Maturities	0	0
Net sales (purchases) of short-term investments	(94)	27
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	0	0
Acquisition of Municipal Assurance Corp., net of cash acquired		(91)
Intercompany debt	0	0
Investment in subsidiary	0	0
Other	0	0
Net cash flows provided by (used in) investing activities	(128)	(65)
Cash flows from financing activities
Proceeds from issuance of common stock		0
Return of capital	0	0
Dividends paid	0	0
Repurchases of common stock	0	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment of long-term debt	0	(173)
Intercompany debt	(7)	173
Net cash flows provided by (used in) financing activities	(7)	0
Effect of exchange rate changes	0	0
Increase (decrease) in cash	(12)	(42)
Cash at beginning of period	13	72
Cash at end of period	1	30
AGMH
Condensed financial statements
Net cash flows provided by (used in) operating activities	68	14
Fixed maturity securities:
Purchases	(23)	(11)
Sales	25	1
Maturities	2	3
Net sales (purchases) of short-term investments	(24)	23
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	0	0
Acquisition of Municipal Assurance Corp., net of cash acquired		0
Intercompany debt	0	0
Investment in subsidiary	50	50
Other	0	0
Net cash flows provided by (used in) investing activities	30	66
Cash flows from financing activities
Proceeds from issuance of common stock		0
Return of capital	0	0
Dividends paid	(98)	(80)
Repurchases of common stock	0	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment of long-term debt	0	0
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(98)	(80)
Effect of exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash at beginning of period	0	0
Cash at end of period	0	0
Other Subsidiaries
Condensed financial statements
Net cash flows provided by (used in) operating activities	207	(82)
Fixed maturity securities:
Purchases	(1,570)	(1,150)
Sales	680	682
Maturities	620	754
Net sales (purchases) of short-term investments	146	266
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	553	407
Acquisition of Municipal Assurance Corp., net of cash acquired		0
Intercompany debt	7	(173)
Investment in subsidiary	0	0
Other	81	85
Net cash flows provided by (used in) investing activities	517	871
Cash flows from financing activities
Proceeds from issuance of common stock		0
Return of capital	(50)	(50)
Dividends paid	(262)	(196)
Repurchases of common stock	0	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	(409)	(553)
Repayment of long-term debt	(22)	(31)
Intercompany debt	0	0
Net cash flows provided by (used in) financing activities	(743)	(830)
Effect of exchange rate changes	(1)	1
Increase (decrease) in cash	(20)	(40)
Cash at beginning of period	125	143
Cash at end of period	105	103
Consolidating Adjustments
Condensed financial statements
Net cash flows provided by (used in) operating activities	(360)	(276)
Fixed maturity securities:
Purchases	65	0
Sales	(65)	0
Maturities	0	0
Net sales (purchases) of short-term investments	0	0
Net proceeds from paydowns on financial guaranty variable interest entities’ assets	0	0
Acquisition of Municipal Assurance Corp., net of cash acquired		0
Intercompany debt	(7)	173
Investment in subsidiary	(50)	(50)
Other	0	0
Net cash flows provided by (used in) investing activities	(57)	123
Cash flows from financing activities
Proceeds from issuance of common stock		0
Return of capital	50	50
Dividends paid	360	276
Repurchases of common stock	0	0
Share activity under option and incentive plans	0	0
Net paydowns of financial guaranty variable interest entities’ liabilities	0	0
Repayment of long-term debt	0	0
Intercompany debt	7	(173)
Net cash flows provided by (used in) financing activities	417	153
Effect of exchange rate changes	0	0
Increase (decrease) in cash	0	0
Cash at beginning of period	0	0
Cash at end of period	$ 0	$ 0