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Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011
--- | --- | --- | --- | ---
Derivative [Line Items] |  |  |  |
Notional Value | $ 1,444.8 | $ 1,432.8 | $ 1,364.1 | $ 1,413.0
Forecasted in 2013 [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Notional Value | $ 110.0 |  |  |
Weighted Average Receive Rate | 6.71% |  |  |
Weighted Average pay rate | 0.28% |  |  |

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