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Employee Incentive Plans (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended | 128 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2017 | May 03, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
401(k) plan contribution expense | $ 3.0 | $ 3.0 | $ 3.0 |  |
Common Stock Shares issued under ROI Unit Stock Plan |  |  |  |  | 1,271,009
ROI Shares issued to IBG LLC |  |  |  |  | 1,250,000
2007 SIP shares issued under ROI Unit Stock Plan |  |  |  |  | 21,009
Fair value ROI Treasury Stock |  |  |  |  | $ 38.0
Common stock to be distributed to former employees under the ROI Unit Stock Plan | 3,849 |  |  |  |
Maximum shares of stock distributable under 2007 Stock Incentive Plan | 30,000,000 |  |  | 30,000,000 |
Shares granted to external directors |  |  |  | 24,263 |
Stock Incentive Plan Granted Shares Adjustment |  | 5,657 |  |  |
2007 Stock Incentive Plan Compensation Expense | $ 53.0 | $ 51.0 | $ 50.0 |  |
Estimated Future 2007 Stock Incentive Plan Compensation Expense | $ 38.0 |  |  | $ 38.0 |
Post employment shares distribution |  |  |  | 573,197 |
2007 Stock Incentive Plan (Shares) |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Percentage | • 10% on the first vesting date, which is on or about May 9 of each year; and• an additional 15% on each of the following six anniversaries of the first vesting, assuming continued employment with the Company and compliance with non-competition and other applicable covenants. |  |  |  |

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- Definition Fair Value of shares issued to IBG LLC, held as treasury stock, in connection with the ROI Dollar Unit Stock Plan. + References No definition available. + Details
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- Definition As of the balance sheet date, the aggregate estimated unrecognized future compensation costs for unvested 2007 Stock Incentive Plan shares. + References No definition available. + Details
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- Definition The maximum number of shares authorized to be granted and issued under the 2007 Stock Incentive Plan + References No definition available. + Details
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- Definition Number of shares to be distributed to former employees under the 2007 ROI Stock Unit Plan. + References No definition available. + Details
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- Definition Number of shares granted to external directors under the 2007 Stock Incentive Plan. + References No definition available. + Details
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- Definition Number of common shares originally registered and held in treasury to be distributed to employees under the ROI Unit Stock Plan. + References No definition available. + Details
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- Definition Number of common shares issued to IBG LLC and held as treasury stock in connection with the investment of accumulated ROI Dollar Unit earnings at the election of employees. Amount includes 21,009 shares allocated from the 2007 Stock Incentive Plan. + References No definition available. + Details
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- Definition Shares allocated from the 2007 Stock Incentive Plan to cover shares issued under the ROI Unit Stock Plan and held as treasury stock. + References No definition available. + Details
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- Definition Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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