Reinsurance Reinsurance - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Insurance [Abstract]
Increase in net unearned premium reserve	$ 0	$ 4	$ 15	$ 64
Increase in net par outstanding	0	224	1,069	1,457
Commutation gains (losses)	$ 0	$ 1	$ 1	$ (16)

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