Acquisitions - Narrative (Details) $ in Millions				1 Months Ended	3 Months Ended		9 Months Ended			21 Months Ended
		Jul. 01, 2016 USD ($)	Apr. 01, 2015 USD ($)	Jan. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)		Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Company		Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Financial guaranty companies acquired | Company										2
GAAP book value					$ 6,640.0		$ 6,640.0			$ 6,640.0			$ 6,063.0
Payment to acquire business							0.0		$ 800.0
Assets					11,445.0		11,445.0			11,445.0			11,192.0
Net par amount outstanding					315,325.0	[1],[2]	315,325.0	[1],[2]		315,325.0	[1],[2]		$ 358,571.0	[3]
CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Cash purchase price	[4]	$ 443.0
Revenue since acquisition date					282.0
Net income since acquisition date					308.0
Net par amount outstanding					3,700.0		3,700.0			3,700.0
Radian [Member]
Business Acquisition [Line Items]
Net par amount outstanding assumed in business acquisition			$ 13,600.0
CIFG Europe S.A. [Member]
Business Acquisition [Line Items]
Assets					43.0		43.0			43.0
Insured Financial Obligations Outstanding Principal Amount, Gross					828.0		828.0			828.0
MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Insured Financial Obligations Outstanding Principal Amount, Gross					$ 347.0		$ 347.0			$ 347.0
Net par amount outstanding												$ 13,000.0
Subsidiaries [Member] | CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Cash purchase price		450.6
Subsidiaries [Member] | Radian [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 804.5
CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Net par amount outstanding assumed in business acquisition		$ 4,200.0
Forecast [Member] | Subsidiaries [Member] | MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Cash expected to be acquired in acquisition				$ 23.0
AGUSH [Member] | CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Ownership percentage by noncontrolling owners		1.60%
Payment to acquire business		$ 7.1

[1] As of September 30, 2016, excludes $2.1 billion of net par as a result of loss mitigation strategies, including loss mitigation securities held in the investment portfolio, which are primarily BIG.

[2] The September 30, 2016 amounts include $3.7 billion of net par acquired from CIFG.

[3] As of December 31, 2015, excludes $1.5 billion of net par as a result of loss mitigation strategies, including loss mitigation securities held in the investment portfolio, which are primarily BIG.

[4] The cash purchase price of $443 million was the cash transferred for the acquisition which was allocated as follows: (1) $270 million for the purchase of net assets of $627 million, and (2) the settlement of pre-existing relationships between CIFG and Assured Guaranty at a fair value of $173 million.