Reinsurance and Other Monoline Exposures - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Insurance [Abstract]
Increase (decrease) in net unearned premium reserve	$ 4	$ 62	$ 64	$ 80
Increase (decrease) in net par outstanding	224	3,455	1,457	4,628
Commutation gains (losses)	$ 1	$ 255	$ (16)	$ 328

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.

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