Income Taxes	6 Months Ended
Jun. 30, 2018
Income Taxes [Abstract]
Income Taxes	11. Income Taxes Income tax expense for the six months ended June 30, 2018 and 2017 differs from the U.S. federal statutory rate primarily due to the taxation treatment of income attributable to noncontrolling interests in IBG LLC and the enactment of the Tax Act, as discussed below. These noncontrolling interests are subject to U.S. taxation as partnerships. Accordingly, the income attributable to these noncontrolling interests is reported in the condensed consolidated statements of comprehensive income, but the related U.S. income tax expense attributable to these noncontrolling interests is not reported by the Company as it is the obligation of the individual members. Income tax expense is also affected by the differing effective tax rates in foreign, state and local jurisdictions where certain of the Company’s subsidiaries are subject to corporate taxation. Deferred income taxes arise primarily due to the amortization of the deferred tax assets recognized in connection with the common stock offerings (see Note 4), differences in the valuation of financial assets and liabilities, and for other temporary differences arising from the deductibility of compensation and depreciation expenses in different time periods for accounting and income tax return purposes. The Tax Act, as previously described, makes broad and complex changes to the U.S. tax code, including, but not limited to, (1) reducing the U.S. federal corporate tax rate to 21%, effective January 1, 2018; (2) requiring a one-time transition tax on certain undistributed earnings of foreign subsidiaries to be paid over eight years; (3) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; (4) requiring a current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations; (5) eliminating the corporate alternative minimum tax (“AMT”) and changing how existing AMT credits can be realized; (6) creating the base erosion anti-abuse tax, a new minimum tax; (7) creating a new limitation on deductible interest expense; (8) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017; (9) repealing the Section 199 manufacturing deduction; and (10) full expensing of qualified property for tax return purposes. The SEC staff issued Staff Accounting Bulletin 118 (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the enactment of the Tax Act for entities to complete the accounting under ASC Topic 740. In accordance with SAB 118, an entity must reflect the income tax effects of those aspects of the Tax Act for which the accounting under ASC Topic 740 is complete. To the extent that an entity’s accounting for certain income tax effects of the Tax Act is incomplete but it is able to determine a reasonable estimate, the entity must record a provisional estimate on its financial statements. However, if an entity cannot determine a provisional estimate to be included on its financial statements, the entity should continue to apply ASC Topic 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the Tax Act. The Company’s accounting for the following elements of the Tax Act is incomplete. However, the Company has made reasonable estimates of certain effects and, therefore, recorded provisional adjustments as follows: Reduction of U.S. federal corporate tax rate: The Tax Act reduces the corporate tax rate to 21%, effective January 1, 2018. For certain of the Company’s deferred tax assets and liabilities, the Company recognized a provisional net decrease of $115 million with a corresponding adjustment to deferred income tax expense (or deferred tax benefit) for the year ended December, 31, 2017. As of June 30, 2018, the Company has not made any adjustments to the provisional amount recorded as of December 31, 2017. While the Company has made a reasonable estimate of the impact of the reduction in corporate rate, it may be affected by other analyses related to the Tax Act, including, but not limited to, the calculation of deemed repatriation of deferred foreign income and the state tax effect of adjustments made to federal temporary differences. The Company is still analyzing certain aspects of the Tax Act and refining its calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts. Deemed Repatriation Transition Tax: The Deemed Repatriation Transition Tax (“Transition Tax”) is a tax on previously untaxed accumulated and current earnings of certain foreign subsidiaries. To determine the amount of the Transition Tax, the Company must determine, in addition to other factors, the amount of post-1986 earnings of the relevant foreign subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. As of December 31, 2017, the Company recognized a provisional Transition Tax obligation of $62 million and for the six months ended June 30, 2018 has not made any adjustments to the provisional amount recorded. This amount may change when the calculation of post-1986 foreign earnings and profits previously deferred from U.S. federal taxation and the amounts held in cash or other specified assets are finalized. The Company does not expect any significant changes, but it is continuing to gather additional information to more precisely compute the amount of the Transition Tax. The Tax Act creates a new requirement that global intangible low taxed income (“GILTI”) earned by controlled foreign corporations (“CFC”s) must be included currently in the gross income of the CFC’s U.S. shareholder. GILTI is the excess of the shareholder’s “net CFC-tested income” over the deemed tangible income return, which is currently defined as the excess of (1) 10 percent of the aggregate of the U.S shareholder’s pro rata share of the qualified business asset investment in each CFC with respect to which it is a U.S shareholder over (2) the amount of certain interest expense taken into account in the determination of net CFC-tested income. Under U.S. GAAP, the Company is allowed to make an accounting policy election of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into the Company’s measurement of its deferred taxes (the “deferred method”). The Company selected the period cost method. As of and for the six months ended June 30, 2018 and 2017, the Company had no unrecognized tax and no valuation allowances on deferred tax assets were required. The Company is subject to taxation in the U.S. and various states and foreign jurisdictions. As of June 30, 2018, the Company is no longer subject to U.S. Federal and State income tax examinations for tax years prior to 2010, and to non-U.S. income tax examinations for tax years prior to 2008. As of June 30, 2018, accumulated earnings held by non‑U.S. subsidiaries totaled $1.1 billion (as of December 31, $1.1 billion). Of this amount, approximately $0.3 billion (as of December 31, 2017 $0.3 billion) is attributable to earnings of the Company’s foreign subsidiaries that are considered “pass‑through” entities for U.S. income tax purposes. Since the Company accounts for U.S. income taxes on these earnings on a current basis, no additional U.S. tax consequences would result from the repatriation of these earnings other than that which would be due arising from currency fluctuations between the time the earnings are reported for U.S. tax purposes and when they are remitted. With respect to certain of these subsidiaries’ accumulated earnings (approximately $0.2 billion and $0.2 billion as of June 30, 2018 and December 31, 2017, respectively), repatriation would result in additional foreign taxes in the form of dividend withholding tax imposed on the recipient of the distribution or dividend distribution tax imposed on the payor of the distribution. The Company has not provided for its proportionate share of these additional foreign taxes as it does not intend to repatriate these earnings in the foreseeable future. For the same reason, the Company has not provided deferred U.S. tax on cumulative translation adjustments associated with these earnings.

11. Income Taxes

Income tax expense for the six months ended June 30, 2018 and 2017 differs from the U.S. federal statutory rate primarily due to the taxation treatment of income attributable to noncontrolling interests in IBG LLC and the enactment of the Tax Act, as discussed below. These noncontrolling interests are subject to U.S. taxation as partnerships. Accordingly, the income attributable to these noncontrolling interests is reported in the condensed consolidated statements of comprehensive income, but the related U.S. income tax expense attributable to these noncontrolling interests is not reported by the Company as it is the obligation of the individual members. Income tax expense is also affected by the differing effective tax rates in foreign, state and local jurisdictions where certain of the Company’s subsidiaries are subject to corporate taxation.

Deferred income taxes arise primarily due to the amortization of the deferred tax assets recognized in connection with the common stock offerings (see Note 4), differences in the valuation of financial assets and liabilities, and for other temporary differences arising from the deductibility of compensation and depreciation expenses in different time periods for accounting and income tax return purposes.

The Tax Act, as previously described, makes broad and complex changes to the U.S. tax code, including, but not limited to, (1) reducing the U.S. federal corporate tax rate to 21%, effective January 1, 2018; (2) requiring a one-time transition tax on certain undistributed earnings of foreign subsidiaries to be paid over eight years; (3) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; (4) requiring a current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations; (5) eliminating the corporate alternative minimum tax (“AMT”) and changing how existing AMT credits can be realized; (6) creating the base erosion anti-abuse tax, a new minimum tax; (7) creating a new limitation on deductible interest expense; (8) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017; (9) repealing the Section 199 manufacturing deduction; and (10) full expensing of qualified property for tax return purposes.

The SEC staff issued Staff Accounting Bulletin 118 (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the enactment of the Tax Act for entities to complete the accounting under ASC Topic 740. In accordance with SAB 118, an entity must reflect the income tax effects of those aspects of the Tax Act for which the accounting under ASC Topic 740 is complete. To the extent that an entity’s accounting for certain income tax effects of the Tax Act is incomplete but it is able to determine a reasonable estimate, the entity must record a provisional estimate on its financial statements. However, if an entity cannot determine a provisional estimate to be included on its financial statements, the entity should continue to apply ASC Topic 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the Tax Act.

The Company’s accounting for the following elements of the Tax Act is incomplete. However, the Company has made reasonable estimates of certain effects and, therefore, recorded provisional adjustments as follows:

Reduction of U.S. federal corporate tax rate: The Tax Act reduces the corporate tax rate to 21%, effective January 1, 2018. For certain of the Company’s deferred tax assets and liabilities, the Company recognized a provisional net decrease of $115 million with a corresponding adjustment to deferred income tax expense (or deferred tax benefit) for the year ended December, 31, 2017. As of June 30, 2018, the Company has not made any adjustments to the provisional amount recorded as of December 31, 2017. While the Company has made a reasonable estimate of the impact of the reduction in corporate rate, it may be affected by other analyses related to the Tax Act, including, but not limited to, the calculation of deemed repatriation of deferred foreign income and the state tax effect of adjustments made to federal temporary differences. The Company is still analyzing certain aspects of the Tax Act and refining its calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts.

Deemed Repatriation Transition Tax: The Deemed Repatriation Transition Tax (“Transition Tax”) is a tax on previously untaxed accumulated and current earnings of certain foreign subsidiaries. To determine the amount of the Transition Tax, the Company must determine, in addition to other factors, the amount of post-1986 earnings of the relevant foreign subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. As of December 31, 2017, the Company recognized a provisional Transition Tax obligation of $62 million and for the six months ended June 30, 2018 has not made any adjustments to the provisional amount recorded. This amount may change when the calculation of post-1986 foreign earnings and profits previously deferred from U.S. federal taxation and the amounts held in cash or other specified assets are finalized. The Company does not expect any significant changes, but it is continuing to gather additional information to more precisely compute the amount of the Transition Tax.

The Tax Act creates a new requirement that global intangible low taxed income (“GILTI”) earned by controlled foreign corporations (“CFC”s) must be included currently in the gross income of the CFC’s U.S. shareholder. GILTI is the excess of the shareholder’s “net CFC-tested income” over the deemed tangible income return, which is currently defined as the excess of (1) 10 percent of the aggregate of the U.S shareholder’s pro rata share of the qualified business asset investment in each CFC with respect to which it is a U.S shareholder over (2) the amount of certain interest expense taken into account in the determination of net CFC-tested income. Under U.S. GAAP, the Company is allowed to make an accounting policy election of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into the Company’s measurement of its deferred taxes (the “deferred method”). The Company selected the period cost method.

As of and for the six months ended June 30, 2018 and 2017, the Company had no unrecognized tax and no valuation allowances on deferred tax assets were required. The Company is subject to taxation in the U.S. and various states and foreign jurisdictions. As of June 30, 2018, the Company is no longer subject to U.S. Federal and State income tax examinations for tax years prior to 2010, and to non-U.S. income tax examinations for tax years prior to 2008.

As of June 30, 2018, accumulated earnings held by non‑U.S. subsidiaries totaled $1.1 billion (as of December 31, $1.1 billion). Of this amount, approximately $0.3 billion (as of December 31, 2017 $0.3 billion) is attributable to earnings of the Company’s foreign subsidiaries that are considered “pass‑through” entities for U.S. income tax purposes. Since the Company accounts for U.S. income taxes on these earnings on a current basis, no additional U.S. tax consequences would result from the repatriation of these earnings other than that which would be due arising from currency fluctuations between the time the earnings are reported for U.S. tax purposes and when they are remitted. With respect to certain of these subsidiaries’ accumulated earnings (approximately $0.2 billion and $0.2 billion as of June 30, 2018 and December 31, 2017, respectively), repatriation would result in additional foreign taxes in the form of dividend withholding tax imposed on the recipient of the distribution or dividend distribution tax imposed on the payor of the distribution. The Company has not provided for its proportionate share of these additional foreign taxes as it does not intend to repatriate these earnings in the foreseeable future. For the same reason, the Company has not provided deferred U.S. tax on cumulative translation adjustments associated with these earnings.