Earnings Per Share - Schedule of EPS (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Common Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 167	$ 34		$ 513	$ 108
Effect of stock compensation plans on allocated net income					11
Net income available to AHL shareholders – diluted		$ 167	$ 34		$ 524	$ 109
Basic weighted average shares outstanding (in shares)		119,519,911	49,798,963		101,506,304	49,960,549
Dilutive effect of stock compensation plans (in shares)		372,358	107,485		3,297,329	92,338
Diluted weighted average shares outstanding (in shares)		119,892,269	49,906,448		104,803,633	50,052,887
Earnings per share
Basic (in USD per share)	[1]	$ 1.40	$ 0.68		$ 5.05	$ 2.18
Diluted (in USD per share)		$ 1.39	$ 0.68		$ 5.00	$ 2.17
Common Class B
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 98	$ 92		$ 443	$ 296
Effect of stock compensation plans on allocated net income					0
Net income available to AHL shareholders – diluted		$ 98	$ 92		$ 443	$ 296
Basic weighted average shares outstanding (in shares)		69,862,355	135,963,975		87,703,973	135,963,975
Dilutive effect of stock compensation plans (in shares)		0	0		0	0
Diluted weighted average shares outstanding (in shares)		69,862,355	135,963,975		87,703,973	135,963,975
Earnings per share
Basic (in USD per share)		$ 1.40	$ 0.68	[1]	$ 5.05	$ 2.18	[1]
Diluted (in USD per share)		$ 1.40	$ 0.68		$ 5.05	$ 2.18
Common Class M-1
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 5			$ 17
Effect of stock compensation plans on allocated net income					0
Net income available to AHL shareholders – diluted		$ 5			$ 17
Basic weighted average shares outstanding (in shares)		3,388,890			3,416,703
Dilutive effect of stock compensation plans (in shares)		0			0
Diluted weighted average shares outstanding (in shares)		3,388,890			3,416,703
Earnings per share
Basic (in USD per share)		$ 1.40			$ 5.05
Diluted (in USD per share)	[1]	$ 1.40			$ 5.05
Common Class M-2
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 1			$ 3
Effect of stock compensation plans on allocated net income					0
Net income available to AHL shareholders – diluted		$ 1			$ 3
Basic weighted average shares outstanding (in shares)		857,831			604,722
Dilutive effect of stock compensation plans (in shares)		7,191			331,206
Diluted weighted average shares outstanding (in shares)		865,022			935,928
Earnings per share
Basic (in USD per share)		$ 1.40			$ 5.05
Diluted (in USD per share)	[1]	$ 1.39			$ 3.26
Common Class M-3
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 1			$ 3
Effect of stock compensation plans on allocated net income					0
Net income available to AHL shareholders – diluted		$ 1			$ 3
Basic weighted average shares outstanding (in shares)		928,870			559,987
Dilutive effect of stock compensation plans (in shares)		289,284			686,268
Diluted weighted average shares outstanding (in shares)		1,218,154			1,246,255
Earnings per share
Basic (in USD per share)		$ 1.40			$ 5.05
Diluted (in USD per share)	[1]	$ 1.07			$ 2.27
Common Class M-4
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income available to AHL shareholders – basic		$ 2			$ 5
Effect of stock compensation plans on allocated net income					0
Net income available to AHL shareholders – diluted		$ 2			$ 5
Basic weighted average shares outstanding (in shares)		1,776,455			1,078,282
Dilutive effect of stock compensation plans (in shares)		1,362,388			1,768,169
Diluted weighted average shares outstanding (in shares)		3,138,843			2,846,451
Earnings per share
Basic (in USD per share)		$ 1.40			$ 5.05
Diluted (in USD per share)	[1]	$ 0.79			$ 1.91

[1] Basic and diluted earnings per share for Class M-1, M-2, M-3 and M-4 were applicable only for the periods ended September 30, 2017. Refer to Note 9 – Earnings Per Share for further discussion.