Investments and Cash - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security	Mar. 31, 2017 USD ($)
Investment [Line Items]
Accrued investment income	$ 101	$ 97
Government agency obligations as a percentage of total mortgage backed securities	42.00%	39.00%
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security	27	28
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	$ 25	$ 27
Assets held-in-trust	$ 264	$ 287
Number of outside managers managing investment portfolio	7
Non-income producing investments	$ 0		$ 0
Unrealized Gain (Loss) on Investments	$ (3)
Investments [Member] | Internally Managed Portfolio [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	12.00%	12.00%
AGL Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust	$ 1,689	$ 1,677
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Investment [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on net income	32
Redeemable Preferred Stock [Member] | Other Income [Member] | Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Investment [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on net income	$ (1)