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Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012
Derivative [Line Items] |  |
Balance at beginning of period | $ 1,432.8 | $ 1,413.0
Additions | 94.0 | 35.0
Terminations | 82.0 | 83.9
Balance at ending of period | 1,444.8 | 1,364.1
Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Additions | 70.0 |
Swaps | Receive Variable/Pay Fixed |  |
Derivative [Line Items] |  |
Balance at beginning of period | 174.0 | 174.0
Additions | 0 | 0
Terminations | 0 | 0
Balance at ending of period | 174.0 | 174.0
Swaps | Receive Fixed/Pay Fixed |  |
Derivative [Line Items] |  |
Balance at beginning of period | 508.8 | 554.0
Additions | 0 | 0
Terminations | 18.0 | 3.9
Balance at ending of period | 490.8 | 550.1
Swaps | Receive Fixed/Pay Variable |  |
Derivative [Line Items] |  |
Balance at beginning of period | 750.0 | 685.0
Additions | 0 | 0
Terminations | 40.0 | 45.0
Balance at ending of period | 710.0 | 640.0
Swaps | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Balance at beginning of period | 0 | 0
Additions | 70.0 | 0
Terminations | 0 | 0
Balance at ending of period | 70.0 | 0
Forwards |  |
Derivative [Line Items] |  |
Balance at beginning of period | 0 | 0
Additions | 24.0 | 35.0
Terminations | 24.0 | 35.0
Balance at ending of period | $ 0 | $ 0

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Notional Amount Of Derivatives, Additions + References No definition available. + Details
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- Definition Notional Amount Of Derivatives, Terminations + References No definition available. + Details
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- Details
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