Variable Interest Entities - Narrative (Details)	1 Months Ended	12 Months Ended
	Feb. 28, 2017 USD ($)	Dec. 31, 2019 USD ($) policy Entity	Dec. 31, 2018 USD ($) policy Entity	Dec. 31, 2017 USD ($) Entity	Jan. 01, 2018 USD ($)	Dec. 31, 2016 Entity
Variable Interest Entity [Line Items]
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt		$ 0
Trustee report general preparation period		30 days
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Total shareholders' equity attributable to AGL		$ 6,639,000,000	$ 6,555,000,000
Fair value gains (losses) on financial guaranty variable interest entities		42,000,000	14,000,000	$ 30,000,000
VIE, Other consolidated, carrying amount, assets		91,000,000	87,000,000
VIE, Other consolidated, carrying amount, liabilities		$ 12,000,000	$ 21,000,000
VIE, Number of policies monitored | policy		18,000
VIE, Number of policies monitored, not within the scope of ASC 810 | policy		16,000
VIE, Number of policies that contain provisions for consolidation | policy		90	110
Variable Interest Entity, Primary Beneficiary [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Unpaid principal balance for FG VIEs’ assets that were 90 days or more past due		$ 52,000,000	$ 71,000,000
Unpaid principal for FG VIEs’ liabilities with recourse		388,000,000	565,000,000
Total shareholders' equity attributable to AGL		(1,000,000)	1,000,000
Fair value gains (losses) on financial guaranty variable interest entities		$ 42,000,000	$ 14,000,000	$ 30,000,000
Deconsolidated | Entity		(3)	(1)	(2)
Variable Interest Entity, Number of Entities | Entity		27	31	32		32
Residential Mortgage Backed Securities and Other Insurance Products [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Change in the instrument specific credit risk of the VIEs' assets		$ 39,000,000	$ 7,000,000	$ 35,000,000
Accounting Standards Update 2016-01 [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Effect of adoption of ASU 2016-01					$ 32,000,000
Net Unrealized Gains (Losses) on FG VIEs’ Liabilities with Recourse due to ISCR [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Total shareholders' equity attributable to AGL		(27,000,000)	$ (31,000,000)	$ 0
Net Unrealized Gains (Losses) on FG VIEs’ Liabilities with Recourse due to ISCR [Member] | Accounting Standards Update 2016-01 [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Effect of adoption of ASU 2016-01					$ 33,000,000
Maximum [Member] | Future Equity Investments [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount	$ 100,000,000
Investment commitment		13,000,000
Assured IM Funds [Member] | Assured Guaranty Subsidiaries [Member] | Equity Method Investments [Member]
Variable Interest Entity [Line Items]
Investments, fair value		77,000,000
Assured IM Funds [Member] | Assured Guaranty Subsidiaries [Member] | Maximum [Member] | Future Equity Investments [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount		500,000,000
Assured IM Funds [Member] | Assured Guaranty Subsidiaries [Member] | Maximum [Member] | Equity Method Investments [Member]
Variable Interest Entity [Line Items]
Long-term purchase commitment, amount		79,000,000
Consolidated Investment Vehicles [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Fair value gains (losses) on financial guaranty variable interest entities		$ (3,000,000)