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Fair Value Inventory Adjustment from Acquisition of MPG - Inventory in process written up to fair value in purchase accounting that was sold in the period. Cost of goods sold (inventory) artificially high due to fair value mark up.

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MPG tax benefit resulting from Tax Cuts and Jobs Act

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

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Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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The amount of identifiable intangible assets recognized as of the acquisition date.

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Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Acquisition of MPG (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 06, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 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
Proceeds from issuance of long-term debt				$ 509.6	$ 2,862.7	$ 30.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				(113.9)	$ 340.0	$ 299.0
Business Acquisition, Pro Forma Earnings Per Share, Diluted					$ 3.51	$ 1.95
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost					$ 20.0
Acquisition-related fair value inventory adjustment				0.0	(24.9)	$ 0.0
Restructuring and acquisition-related costs				78.9	110.7	26.2
Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Percentage of equity interests acquired	100.00%
Total consideration transferred		$ 1,530.2			1,530.2
Net debt assumed					1,700.0
Debt acquired	$ 1,918.7
Cash and cash equivalents acquired	202.1
Proceeds from Notes Payable					1,200.0
Goodwill, Purchase Accounting Adjustments				0.9
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
Term Loan A [Member] \| Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Proceeds from issuance of long-term debt	100.0
Term Loan B [Member] \| Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Proceeds from issuance of long-term debt	$ 1,550.0
Unsecured Debt [Member] \| 6.25% Notes due 2025 [Member]
Business Acquisition [Line Items]
Long-term debt			700.0	$ 700.0	700.0		$ 700.0
Debt Instrument, Interest Rate, Stated Percentage				6.25%			6.25%
Unsecured Debt [Member] \| 6.25% Notes due 2025 [Member] \| Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Long-term debt							$ 700.0
Debt Instrument, Interest Rate, Stated Percentage							6.25%
Unsecured Debt [Member] \| 6.50% Notes [Member]
Business Acquisition [Line Items]
Long-term debt			$ 500.0	$ 500.0	$ 500.0		$ 500.0
Debt Instrument, Interest Rate, Stated Percentage				6.50%			6.50%
Unsecured Debt [Member] \| 6.50% Notes [Member] \| Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Long-term debt							$ 500.0
Debt Instrument, Interest Rate, Stated Percentage							6.50%