Regulatory Capital (Schedule Of Regulatory Capital Amount And Ratios) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Parent Company [Member]	Dec. 31, 2012 Parent Company [Member]	Mar. 31, 2013 Parent Company [Member] Minimum [Member]	Mar. 31, 2013 Parent Company [Member] Maximum [Member]	Mar. 31, 2013 Ally Bank [Member]	Dec. 31, 2012 Ally Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
assets greater than $50 billion	$ 50,000,000,000
Tier 1 common ratio above 5 percent	5.00%
Tier 1 (to risk-weighted assets), Amount		20,663,000,000	20,232,000,000			14,380,000,000	14,136,000,000
Tier 1 (to risk-weighted assets), Ratio		14.59%	13.13%			16.68%	16.26%
Tier 1 (to risk-weighted assets), Required minimum		4.00%				4.00%
Tier 1 (to risk-weighted assets), Well-capitalized minimum		6.00%				6.00%
Total (to risk-weighted assets), Amount		22,084,000,000	21,669,000,000			15,073,000,000	14,827,000,000
Total (to risk-weighted assets), Ratio		15.59%	14.07%			17.48%	17.06%
Total (to risk-weighted assets), Required minimum		8.00%				8.00%
Total (to risk-weighted assets), Well-capitalized minimum		10.00%				10.00%
Tier 1 leverage (to adjusted average assets), Amount		20,663,000,000	20,232,000,000			14,380,000,000	14,136,000,000
Tier 1 leverage (to adjusted average assets), Ratio		12.01%	11.16%			15.59%	15.30%
Tier One Leverage Capital Required for Capital Adequacy to Average Assets				3.00%	4.00%
Tier 1 Leverage To Average Assets For Ally Bank						15.00%
Tier 1 leverage (to adjusted average assets), Well-capitalized minimum						5.00%
Tier 1 common (to risk-weighted assets), Amount		$ 11,180,000,000	$ 10,749,000,000
Tier 1 common (to risk-weighted assets), Ratio		7.89%	6.98%