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Significant Accounting Polices - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Period of forbearance within twenty-four months (more than) | 3 months |  |
Period after grace period for forbearance allowance | 9 months |  |
Forbearance period after grace period (up to) | 6 months |  |
Percentage of loans granted forbearance qualified as TDR | 69.00% |  | 69.00%
Notional value | $ 5,900.0 |  |
Excess tax benefit, amount | 7.3 |  |
Chicago Mercantile Exchange |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Notional value | $ 5,100.0 |  |
Percent of total notional derivative contracts | 85.80% |  |
London Clearing House |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Notional value | $ 700.0 |  |
Percent of total notional derivative contracts | 11.90% |  |
Accounting Standards Update 2016-09 | Retained Earnings | New Accounting Pronouncement, Early Adoption, Effect |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Cumulative effect adjustment to retained earnings |  | $ 0.2 |

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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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