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Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Derivative |  |
Balance at beginning of period | $ 904.5 | $ 1,052.0
Additions | 46.7 | 0.0
Terminations | 70.0 | 30.4
Balance at ending of period | 881.2 | 1,021.6
Credit Risk Contract | Credit Default Swaps |  |
Derivative |  |
Balance at beginning of period | 70.0 | 70.0
Additions | 0.0 | 0.0
Terminations | 70.0 | 0.0
Balance at ending of period | 0.0 | 70.0
Forwards |  |
Derivative |  |
Balance at beginning of period | 0.0 | 10.0
Additions | 26.8 | 0.0
Terminations | 0.0 | 10.0
Balance at ending of period | 26.8 | 0.0
Receive Variable/Pay Fixed | Interest Rate Swaps |  |
Derivative |  |
Balance at beginning of period | 48.0 | 105.5
Additions | 0.0 | 0.0
Terminations | 0.0 | 0.1
Balance at ending of period | 48.0 | 105.4
Receive Fixed/Pay Fixed | Interest Rate Swaps |  |
Derivative |  |
Balance at beginning of period | 536.5 | 616.5
Additions | 19.9 | 0.0
Terminations | 0.0 | 20.3
Balance at ending of period | 556.4 | 596.2
Receive Fixed/Pay Variable | Interest Rate Swaps |  |
Derivative |  |
Balance at beginning of period | 250.0 | 250.0
Additions | 0.0 | 0.0
Terminations | 0.0 | 0.0
Balance at ending of period | $ 250.0 | $ 250.0

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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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