Major Items Causing Income Tax Provision to Differ from South African Mining Statutory Rate (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
Dec. 31, 2010	Dec. 31, 2011
Income Taxes [Line Items]
Non taxable income/non deductible expenditure, share-based-compensation	$ 11.6	$ 24.5
Non taxable income/non deductible expenditure, exploration, feasibility and evaluation costs	25.9	92.8
Non taxable income/non deductible expenditure, national stabilization	12.6	35.9
Impact of royalty expense on product sales, predetermined percentage	5.00%
Broad-based BEE consortium (BEECO)
Income Taxes [Line Items]
Non taxable income/non deductible expenditure, share-based-compensation	$ 128.0

The portion of the difference between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to differences in the deductibility of incentive stock options and other equity-based compensation costs in accordance with generally accepted accounting principles and enacted tax laws.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration