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Employee Benefit Plans - Additional Information (Detail) |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 15, 2017 shares | Dec. 01, 2017 shares | May 19, 2017 USD ($) shares | Dec. 31, 2017 USD ($) Pension_Plan shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Equity incentive plan termination date |  |  |  | Mar. 03, 2027 |  |
Stock options, total intrinsic value | $ |  |  |  | $ 400,000 | $ 1,600,000 | $ 13,100,000
Cash received from stock option exercises | $ |  |  |  | 400,000 | 900,000 | 7,500,000
Tax benefit from stock options exercised | $ |  |  |  | 100,000 | 400,000 | 3,200,000
Defined benefit plan | $ |  |  |  | $ 286,100,000 | $ 248,100,000 | $ 231,900,000
Percent of 401(k) that can be invested in common stock |  |  |  | 25.00% |  |
Number of share held as investment under 401(k) Plan |  |  |  | 1,200,000 |  |
UNITED STATES [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Company contribution percent per year per hours worked |  |  |  | 20.00% |  |
Amount of hours worked for matched contributions |  |  |  | 1000 hours |  |
Percent of annual compensation match percentage |  |  |  | 50.00% | 50.00% | 50.00%
Percent of annual compensation to be matched |  |  |  | 6.00% | 6.00% | 5.00%
Defined benefit plan, annual compensation expense maximum | $ |  |  |  | $ 150,000 | $ 150,000 | $ 150,000
Defined contribution plan, expenses recognized | $ |  |  |  | $ 38,800,000 | 44,300,000 | $ 29,000,000
United Kingdom [Member] | Pension Plan [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of contributory defined benefit pension plans in the United Kingdom | Pension_Plan |  |  |  | 2 |  |
Fair value of plan assets | $ |  |  |  | $ 333,500,000 | 286,600,000 |
Benefit obligation | $ |  |  |  | 455,600,000 | 416,900,000 |
Funded status | $ |  |  |  | 122,100,000 | 130,300,000 |
Unamortized actuarial loss | $ |  |  |  | $ 194,300,000 | $ 209,600,000 |
Time Based Vesting [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Term of equity incentive and stock option plans |  |  |  | 4 years |  |
Time-vesting awards, portion to be vested per year |  |  |  | 25.00% |  |
Stock Options [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Granted options |  |  |  | 0 | 0 | 0
Restricted Stock Units [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, measurement period |  |  |  | 6 years |  |
Restricted Stock Units [Member] | Time Based Vesting [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  |  |  | 1,466,986 | 1,436,310 | 1,535,940
Restricted Stock Units [Member] | Time Based Vesting [Member] | Senior Brokers [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, measurement period | 3 years |  |  |  |  |
Granted, Shares / Units | 127,160 |  |  |  |  |
Restricted Stock Units [Member] | Performance Based Vesting [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  |  |  | 1,458,033 | 60,098 | 1,281,267
Non-Vested Stock Awards [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Granted, Shares / Units |  |  |  | 5,152,082 | 1,496,408 | 2,195,638
Stock options, compensation expense | $ |  |  |  | $ 93,100,000 | $ 63,500,000 | $ 74,700,000
Stock options, unrecognized estimated compensation cost | $ |  |  |  | $ 243,300,000 |  |
Weighted average period of recognition |  |  |  | 3 years 9 months 18 days |  |
Maximum [Member] | UNITED STATES [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Employee subscription rate |  |  |  | 75.00% |  |
Minimum [Member] | UNITED STATES [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Employee subscription rate |  |  |  | 1.00% |  |
2004 Stock Incentive Plan [Member] | Class A Common Stock [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock incentive plans, common stock outstanding |  |  |  | 5,658 |  |
2017 Equity Incentive Plan [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Unissued common stock available for issuance |  |  | 0 |  |  |
Maximum number of share to be granted per person |  |  | 3,300,000 |  |  |
Maximum amount of share to be granted per person | $ |  |  | $ 5,000,000 |  |  |
2017 Equity Incentive Plan [Member] | Stock Options [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  |  |  | 5,573,842 |  |
2017 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of trading days required for purposes of measuring shareholder return based on average closing price of common stock immediately preceding grant date |  | 60 days |  |  |  |
2017 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | Time Based Vesting [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Sharebased compensation arrangement by sharebased payment percentage of target restricted stock unit |  | 33.30% |  |  |  |
2017 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | Total Shareholder Return [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Equity incentive plan termination date |  | Dec. 01, 2023 |  |  |  |
Sharebased compensation arrangement by sharebased payment percentage of target restricted stock unit |  | 33.30% |  |  |  |
Share-based compensation arrangement by share-based payment award, measurement period |  | 6 years |  |  |  |
2017 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | EPS [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Equity incentive plan termination date |  | Dec. 31, 2023 |  |  |  |
Sharebased compensation arrangement by sharebased payment percentage of target restricted stock unit |  | 33.30% |  |  |  |
Share-based compensation arrangement by share-based payment award, measurement period |  | 6 years |  |  |  |
2017 Equity Incentive Plan [Member] | Maximum [Member] | Employee Stock Options And Stock Appreciation Rights [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Term of equity incentive and stock option plans |  |  | 10 years |  |  |
2017 Equity Incentive Plan [Member] | Maximum [Member] | Restricted Stock Units [Member] | Time Based Vesting [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  | 939,605 |  |  |  |
2017 Equity Incentive Plan [Member] | Maximum [Member] | Restricted Stock Units [Member] | Performance Based Vesting [Member] | Employees [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  | 3,288,618 |  |  |  |
2017 Equity Incentive Plan [Member] | Minimum [Member] | Time Based Vesting [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Term of equity incentive and stock option plans |  |  | 3 years |  |  |
2017 Equity Incentive Plan [Member] | Class A Common Stock [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares reserved for future issuance |  |  | 10,000,000 |  |  |
2012 Equity Incentive Plan [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares available for future grant |  |  |  | 5,829,189 |  |
2017 or 2012 Equity Incentive Plans [Member] | Stock Options [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock option exchange, shares issued |  |  |  | 0 |  |
2017 or 2012 Equity Incentive Plans [Member] | Maximum [Member] | Stock Options [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock options, expected term |  |  |  | 7 years |  |
2017 or 2012 Equity Incentive Plans [Member] | Minimum [Member] | Stock Options [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock options, expected term |  |  |  | 5 years |  |

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- Definition Number of shares of the reporting entity held as investments by employees in the reporting entity's defined benefit plan. + References No definition available. + Details
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- Definition Number of trading term required for purposes of measuring shareholder return based on average closing price of common stock immediately preceding grant date. + References No definition available. + Details
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- Definition The percentage of an employee's investment that can be invested in the reporting entity's common stock. + References No definition available. + Details
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- Definition The percentage of the employee's annual compensation that is eligible for the company match. + References No definition available. + Details
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- Definition The percentage the defined benefit plan participant vests in the company match based on years employed by the reporting entity assuming they work for at least a minimum stated number of hours in each year. + References No definition available. + Details
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- Definition The number of unissued shares under the reporting entity's previous stock plan that remain available to issue under the reporting entity's current stock plan. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919 + Details
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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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- Definition Maximum amount the employee may contribute to a defined contribution plan. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Date the equity-based award expires, in CCYY-MM-DD format. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The highest quantity of shares an employee can purchase under the plan per period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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