Consolidated Statements of Operations Consolidated Statement of Operations - Parenthetical - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Gain from commutations	$ 328	$ 8	$ 28

Represents the gains (losses) from commutations to recapture business previously ceded to other insurers.

No definition available.

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