Acquisitions - Narrative (Details) $ in Millions				6 Months Ended		9 Months Ended		12 Months Ended					24 Months Ended
	Jan. 10, 2017 USD ($)	Jul. 01, 2016 USD ($)	Apr. 01, 2015 USD ($)	Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)		Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) Company
Business Acquisition [Line Items]
Number of business acquired | Company													3
Acquisition of Radian Asset, net of cash acquired								$ 0.0		$ 800.0		$ 0.0
Investments				$ 10,985.0		$ 11,192.0		10,985.0		11,192.0			$ 10,985.0
Gross Par Outstanding				307,474.0		373,192.0		307,474.0		373,192.0			307,474.0
Net par amount outstanding				296,318.0	[1]	358,571.0	[2]	296,318.0	[1]	358,571.0	[2]		296,318.0	[1]
CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Cash purchase price		$ 443.0
Revenue				307.0
Net income (loss)				323.0
Net par amount outstanding				2,900.0				2,900.0					2,900.0
Radian [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 804.0
Net par amount outstanding assumed in business acquisition			13,600.0
Revenue						560.0
Net income (loss)						366.0
Net par amount outstanding						$ 10,900.0				$ 10,900.0
CIFG Europe S.A. [Member]
Business Acquisition [Line Items]
Investments				41.0				41.0					41.0
Gross Par Outstanding				694.0				694.0					694.0
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
MBIA UK [Member]
Business Acquisition [Line Items]
Net par amount outstanding				$ 12,000.0				$ 12,000.0					$ 12,000.0
AGUS [Member] | CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Ownership percentage by noncontrolling owners		1.60%
Acquisition of Radian Asset, net of cash acquired		$ 7.1
AGUS [Member] | Subsidiaries [Member] | Radian [Member]
Business Acquisition [Line Items]
Acquisition loan from parent company			$ 200.0
Subsequent Event [Member] | Subsidiaries [Member] | MBIA UK [Member]
Business Acquisition [Line Items]
Cash Acquired from Acquisition	$ 23.0
Business Combination, Consideration Transferred, Bond Exchanged Amount	347.0
Gross Par Outstanding, Fair Value	$ 334.0

[1] The December 31, 2016 amounts include $2.9 billion of net par from the CIFG Acquisition.

[2] The December 31, 2015 amounts include $10.9 billion of net par from the Radian Asset Acquisition.