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Financial Instruments - Summary of Coverage Ratio and Leverage Ratio (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about financial instruments [line items] |  |  |
Coverage ratio | 3.46 | 3.18 | 2.61
Leverage ratio | 3.85 | 4.22 | 5.21
Less than or Equal to 5.25 Ratio [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Leverage ratio | 5.25 |  |
Less than or Equal to 5.00 Ratio [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Leverage ratio | 5.00 |  |
Less than or equal to four point seven five ratio [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Leverage ratio | 4.75 |  |
Less than or Equal to 4.50 Ratio [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Leverage ratio | 4.50 |  |
Less than or Equal to 4.25 Ratio [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Leverage ratio | 4.25 |  |
Greater than or Equal to 2.50 [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Coverage ratio | 2.50 |  |
Greater than or Equal to 2.75 [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Coverage ratio | 2.75 |  |

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- Definition Coverage ratio. + References No definition available. + Details
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- Definition Leverage ratio. + References No definition available. + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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