AAM Common Stock Issued per MPG Common Stock as Part of the Merger Consideration

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Adjustment to Contractual Purchase Price - Existing Payable Balance

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Business Combination, Net Debt to be Assumed by Acquiring Entity. Gross Debt Assumed Less Cash Acquired.

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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 Pro Forma Net Income Adjustment

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MPG tax benefit resulting from Tax Cuts and Jobs Act

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Costs incurred for restructuring initiatives as well as expenses directly related to acquisition activity.

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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No definition available.

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

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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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This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to financial liabilities assumed in connection with a business combination for which the initial accounting was incomplete.

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This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to intangibles acquired in connection with a business combination for which the initial accounting was incomplete.

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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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- Definition

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 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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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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The cash inflow from a borrowing supported by a written promise to pay an obligation.

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Additional share-based compensation cost recognized as a result of an occurrence of an event that accelerates its recognition.

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Business Combinations 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	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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					$ 2,862.7	$ 30.3	$ 16.8
Business combination, provisional information, initial accounting incomplete, adjustment, intangibles				$ 16.2
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest					340.0	$ 299.0	272.7
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost					$ 20.0
Business Acquisition, Pro Forma Earnings Per Share, Diluted					$ 3.51	$ 1.95
Acquisition-related fair value inventory adjustment					$ (24.9)	$ 0.0	0.0
Restructuring and acquisition-related costs					110.7	26.2	$ 0.0
Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Percentage of equity interests acquired	100.00%
Total consideration transferred		$ 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
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 Acquisition, Pro Forma Net Income (Loss)				320.0	400.0	220.0
Business Acquisition, Pro Forma Revenue					7,000.0	6,600.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				93.5
Acquisition-related fair value inventory adjustment					25.0
Restructuring and acquisition-related costs					55.0
MPG Pro Forma Net Income Adjustment					65.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual					227.0
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	0.0
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	$ 0.0
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%	6.50%
Capital Lease Obligations [Member] \| Metaldyne Performance Group, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities					$ 23.0