Reinsurance and Other Monoline Exposures - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Increase (decrease) in net unearned premium reserve	$ 82	$ 0	$ 23
Increase (decrease) in net par outstanding	5,107	28	855
Commutation gains (losses)	$ 328	$ 8	$ 28

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