Reinsurance and Other Monoline Exposures - Reinsurance and Other Monoline Exposures (Details) $ in Millions	Jun. 30, 2016 USD ($)
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	$ 70
Ceded Premium, net of Commissions	(50)
Assumed Expected Loss to be Paid	(151)
Ceded Expected Loss to be Paid	102
American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(5)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	29
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(11)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	46
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	13
Ceded Premium, net of Commissions	(20)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	8
Mitsui Sumitomo Insurance Co. Ltd. [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(2)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	19
Ambac Assurance Corporation [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	36
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(3)
Ceded Expected Loss to be Paid	0
Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	7
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(45)
Ceded Expected Loss to be Paid	0
CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0	[1]
Ceded Premium, net of Commissions	0	[1]
Assumed Expected Loss to be Paid	(71)	[1]
Ceded Expected Loss to be Paid	0	[1]
MBIA [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	4
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(9)
Ceded Expected Loss to be Paid	0
National [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	6
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(6)
Ceded Expected Loss to be Paid	0
FGIC [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	4
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(17)
Ceded Expected Loss to be Paid	0
Other [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(12)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	$ 0

[1] On July 1, 2016, AGC acquired all of the issued and outstanding capital stock of CIFG Holding Inc., the parent of financial guaranty insurer CIFG. On July 5, 2016, CIFG merged with and into AGC, with AGC as the surviving company. See Note 2, Acquisitions, for additional information