Insurance Company Regulatory Requirements - Insurance Regulatory Amounts Reported (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AGM
Statutory Accounting Practices [Line Items]
Policyholders' Surplus	$ 2,864	$ 2,691
Net Income (Loss)	374	312	$ 172
Statutory capital and surplus required	828	869
AGC
Statutory Accounting Practices [Line Items]
Policyholders' Surplus	1,717	1,775
Net Income (Loss)	73	226	(5)
Statutory capital and surplus required	545	546
MAC
Statutory Accounting Practices [Line Items]
Policyholders' Surplus	305	276
Net Income (Loss)	37	53	55
Statutory capital and surplus required	184	192
AG Re
Statutory Accounting Practices [Line Items]
Policyholders' Surplus	1,026	1,098
Net Income (Loss)	24	45	131
AGRO
Statutory Accounting Practices [Line Items]
Policyholders' Surplus	429	410
Net Income (Loss)	$ 7	$ 12	$ 10
Subsidiaries | AGM | MAC
Statutory Accounting Practices [Line Items]
Holding company's percent ownership of common stock	60.70%
Subsidiaries | AGC | MAC
Statutory Accounting Practices [Line Items]
Holding company's percent ownership of common stock	39.30%
Subsidiaries | MAC | MAC
Statutory Accounting Practices [Line Items]
Holding company's percent ownership of common stock	100.00%