Business Combinations - Acquisition of MPG Narrative (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 06, 2017
Business Acquisition [Line Items]
Business Acquisition, Share Price							$ 13.50
AAM Common Stock Issued per MPG Common Stock as Part of the Merger Consideration							0.5
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest			$ (533.0)	$ (113.9)	$ 340.0
Business Acquisition, Pro Forma Earnings Per Share, Diluted					$ 3.51	$ 1.95
Share-based Payment Arrangement, Accelerated Cost					$ 20.0
Acquisition-related fair value inventory adjustment			0.0	0.0	(24.9)
Restructuring and acquisition-related costs			$ 57.8	$ 78.9	110.7
Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Percentage of equity interests acquired							100.00%
Total consideration transferred	$ 1,530.2				1,500.0
Net debt assumed					1,700.0
Debt acquired							$ 1,918.7
Cash and cash equivalents acquired							202.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles							1,223.1
Adjustment to Contractual Purchase Price - Existing Payable Balance							$ (12.4)
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual		$ 2,022.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual		320.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual					227.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest		$ 93.5
Business Acquisition, Pro Forma Revenue					7,000.0	$ 6,600.0
Business Acquisition, Pro Forma Net Income (Loss)					400.0	$ 220.0
Acquisition-related fair value inventory adjustment					25.0
Restructuring and acquisition-related costs					55.0
MPG Pro Forma Net Income Adjustment					$ 65.0