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Common Shares - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 1 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 31, 2010 | Dec. 31, 2012 Vote | Dec. 31, 2011 | Dec. 31, 2010 | Nov. 10, 2012 | Nov. 10, 2011 | Nov. 09, 2011 | Nov. 11, 2010
Preference shares included in authorized Common Shares |  | 99,760,000 |  |  |  |  |  |
Shares issued or outstanding |  | 0 |  |  |  |  |  |
Number of votes per share |  | 1 |  |  |  |  |  |
Number of common stock shares to be purchased under the Bid |  |  |  |  |  |  | 12,000,000 | 8,000,000
Commencement date of Bid |  | November 13, 2012 | November 11, 2011 | November 11, 2010 |  |  |  |
Termination date of Bid |  | November 12, 2013 | November 10, 2012 | November 10, 2011 |  |  |  |
Number of Common Shares purchased under the Bid |  | 427,402 |  |  | 3,668,430 | 8,000,000 |  |
Percentage of issued and outstanding common shares |  | 5.20% | 5.10% |  |  |  |  |
Common Shares purchased for cancellation |  | 895,032 | 10,747,300 |  |  |  |  |
Cash paid |  | $ 40 | $ 407 | $ 23 |  |  |  |
Cost for plan of arrangement | 10 |  |  |  |  |  |  |
Stronach Group [Member] |  |  |  |  |  |  |  |
Equity ownership and voting interest | 7.40% |  |  |  |  |  |  |
Class B Share Repurchase [Member] |  |  |  |  |  |  |  |
Cash paid | 300 |  |  |  |  |  |  |
Number of shares repurchased | 1,453,658 |  |  |  |  |  |  |
Class A Subordinate Voting Share Issuance [Member] |  |  |  |  |  |  |  |
Issuance of Class A Subordinate Voting Shares | $ 18.0 |  |  |  |  |  |  |

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