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Derivative Instruments - Additional Information (Detail) (Interest rate swaps [Member], USD $)
In Millions, unless otherwise specified |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
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Jun. 30, 2013
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Jun. 30, 2012
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Jun. 30, 2013
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Jun. 30, 2012
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Interest rate swaps [Member]
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| Derivative [Line Items] | ||||
| Interest rate swaps realized losses | $ 2.5 | $ 2.4 | $ 4.9 | $ 4.8 |
| Interest rate swaps unrealized gains (losses) | 5.3 | (1.5) | 6.9 | (0.2) |
| Aggregate fair value liability | $ 11.0 | $ 11.0 | ||