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	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Insurance [Abstract]
Increase (decrease) in net unearned premium reserve	$ 56	$ 0	$ 60	$ 18
Increase (decrease) in net par outstanding	1,191	0	1,233	1,173
Commutation gains (losses)	$ (18)	$ 0	$ (17)	$ 73

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