Reinsurance and Other Monoline Exposures - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Insurance [Abstract]
Increase (decrease) in net unearned premium reserve	$ 20	$ 11	$ 109
Increase (decrease) in net par outstanding	1,167	151	19,173
Commutation gains recorded in other income	$ 23ago_GainLossFromCommutations	$ 2ago_GainLossFromCommutations	$ 82ago_GainLossFromCommutations

Change during the period in the unearned portion of premiums written resulting from commutations to recapture business previously ceded to other insurers.

No definition available.

Name:	ago_IncreaseDecreaseInUnearnedPremiumsDueToCommutations
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration