Share-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2010
Employee Share Purchase Plan [Abstract]
Employee Share Purchase Plan Description	Towers Watson assumed the amended and restated Watson Wyatt 2001 Employee Stock Purchase Plan (the “Stock Purchase Plan”) which enables associates to purchase shares of Towers Watson Class A common stock at a 5% discount. The Stock Purchase Plan is a non-compensatory plan under generally accepted accounting principles of stock-based compensation. As a result, no compensation expense is recognized in conjunction with this plan.
Employee Share Purchase Plan Shares Issued During Period	56,000	108,000	146,000
Restricted Stock Units Information
Description of Restricted Stock Units	In September 2010, the Compensation Committee of our Board of Directors approved the form of performance-vested restricted stock unit award agreement, pursuant to the 2009 Plan. RSUs are designed to provide us an opportunity to offer long-term incentives and to provide key executives with a long-term stake in our success. RSUs are notional, non-voting units of measurement based on our common stock. Under the RSU agreement, participants become vested in a number of RSUs based on the achievement of specified levels of financial performance during the performance period set forth in the agreement, provided that the participant remains in continuous service with us through the end of the performance period. The targets reflect an emphasis on continued profitability growth through successful integration, despite the difficult economic environment. Any RSUs that become vested are payable in shares of our Class A Common Stock. Dividend equivalents will accrue on RSUs and vest to the same extent as the underlying shares. In September 2011, the Compensation Committee amended the form of performance-vested restricted stock unit award agreement to include a provision whereby the Committee could provide for continuation of vesting of restricted stock units upon an employee's termination under certain circumstances such as a qualified retirement. This definition of qualified retirement (age 55 and with 15 years experience at the Company and a minimum of one year of service in the performance period) was further defined in January 2012 and the amended award agreements for all of our outstanding LTIP awards were finalized and distributed to participants.
Restricted Stock Units granted during the period	700,000	186,000
Restricted Stock Units vested in the period	432,000	32,000
Restricted Stock Units, balance not yet vested	438,000	173,000	24,000
Allocated Share-based Compensation Expense	$ 54.5	$ 78.0	$ 57.9
Modification description	This definition of qualified retirement (age 55 and with 15 years experience at the Company and a minimum of one year of service in the performance period) was further defined in January 2012 and the amended award agreements for all of our outstanding LTIP awards were finalized and distributed to participants.
Share Based Compensation Expense Not Yet Recognized	10.9
Remaining weighted average contractual life of restricted stock units	1 year 4 months
Restricted stock unit rollforward of number of shares
Nonvested as at, shares	173,000	24,000
Granted, shares	700,000	186,000
Vested, shares	(432,000)	(32,000)
Forfeited, shares	(3,000)	(5,000)
Nonvested and expected to vest, as at, shares	438,000	173,000	24,000
Restricted stock unit rollforward of weighted average fair value
Nonvested as at, grant date fair value	$ 46.31	$ 48.98
Granted, grant date fair value	$ 58.55	$ 46.07
Vested, grant date fair value	$ 57.69	$ 47.54
Forfeited, grant date fair value	$ 57.43	$ 44.69
Nonvested and expected to vest as at, grant date fair value	$ 54.57	$ 46.31	$ 48.98
Maximum [Member]
Restricted Stock Units Information
Closing stock price on date of grant	$ 63.94
Select Equity Plan [Member]
Restricted Stock Units Information
Description of Restricted Stock Units	During the first quarter of fiscal year 2012, we granted 577,190 equity awards to certain employees under our Select Equity Plan, of which 288,595 were issued as Class A common stock in conjunction with our annual fiscal year 2011 performance bonus. The remaining 288,595 were issued as RSUs that will vest annually over a three-year period. We assumed a 10% forfeiture rate for the RSUs which was adjusted to 5% based on the first year of experience.
Restricted Stock Units granted during the period	577,190
Restricted Stock Units vested in the period	95,045
Restricted Stock Units, balance not yet vested	288,595
Forfeiture rate	10.00%
Allocated Share-based Compensation Expense	11.1
Adjusted forfeiture rate	5.00%
Restricted stock unit rollforward of number of shares
Granted, shares	577,190
Vested, shares	(95,045)
Nonvested and expected to vest, as at, shares	288,595
Select Equity Plan [Member] | Common Class A [Member]
Restricted Stock Units Information
Shares issued during the period for share based compensation	288,595
2012 LTIP
Restricted Stock Units Information
Description of Restricted Stock Units	Awards are based on the value of the executive officer's annual base salary and a multiplier, which is then converted into a target number of RSUs based on our closing stock price as of the date of grant which was between $63.73 and $63.94. Between 0% and 204% of the target number of RSUs will vest based on the extent to which specified performance metrics are achieved over the three-year performance period from July 1, 2011 to June 30, 2014, subject to their continued employment with us through the end of the performance period, except in the case of a qualified retirement. The Compensation Committee approved the grants and established adjusted EBITDA margin and revenue growth during the performance period as the performance metrics for the awards. We accelerated the expense for participants that had achieved a qualified retirement requirement and recorded the remaining non-cash stock based compensation for their awards as if their service requirement has been achieved. We will adjust the stock-based compensation for their awards during the performance period based upon the level of performance achieved.
Restricted Stock Units granted during the period	86,188
Allocated Share-based Compensation Expense	5.7
Restricted stock unit rollforward of number of shares
Granted, shares	86,188
2012 LTIP | Minimum [Member]
Restricted Stock Units Information
Closing stock price on date of grant	$ 63.73
Other Employees [Member]
Restricted Stock Units Information
Description of Restricted Stock Units	granted to certain employees and vest in equal installments over a three-year period based on continued employment through the vesting period.
Restricted Stock Units granted during the period	16,239	28,500
Restricted Stock Units vested in the period	1,575	7,200
Allocated Share-based Compensation Expense	0.7	1.5
Restricted stock unit rollforward of number of shares
Granted, shares	16,239	28,500
Vested, shares	(1,575)	(7,200)
Outside Directors [Member]
Restricted Stock Units Information
Description of Restricted Stock Units	the annual award for outside directors for service on the board of directors in equal quarterly installments over fiscal year 2012	RSUs were granted for the annual award for outside directors for service on the board of directors in equal quarterly installments over fiscal year 2011	RSUs were granted for the initial award for outside directors for service on the board of directors which vest in equal annual installments over a three year period ending January 1, 2013.
Restricted Stock Units granted during the period	12,783	24,710	22,149
Allocated Share-based Compensation Expense	1.0	1.3
Restricted stock unit rollforward of number of shares
Granted, shares	12,783	24,710	22,149
2011 LTIP
Restricted Stock Units Information
Description of Restricted Stock Units	Awards are based on the value of the executive officer’s annual base salary and a multiplier, which is then converted into a target number of RSUs based on our closing stock price as of the date of grant of $45.25. Between 0% and 204% of the target number of RSUs will vest based on the extent to which specified performance metrics are achieved over the three-year performance period from July 1, 2010 to June 30, 2013, subject to their continued employment with us through the end of the performance period. The Compensation Committee approved the grants and established adjusted EBITDA margin for the six-month period ending June 30, 2013 and revenue growth during the performance period (based on fiscal year 2013 revenue versus fiscal year 2010 revenue) as the performance metrics for the awards.
Restricted Stock Units granted during the period		125,192
Allocated Share-based Compensation Expense	$ 5.5	$ 3.2
Closing stock price on date of grant		$ 45.25
Restricted stock unit rollforward of number of shares
Granted, shares		125,192
Towers Perrin Merger [Member]
Restricted Stock Units Information
Restricted Stock Units granted during the period			4,248,984
Restricted stock unit rollforward of number of shares
Granted, shares			4,248,984
Outside Directors Shorter Vesting [Member]
Restricted Stock Units Information
Restricted Stock Units granted during the period			9,844
Restricted stock unit rollforward of number of shares
Granted, shares			9,844
Extend Health Options Assumed [Member]
Shares registered and available for issuance
Shares registered	377,614
Extend Health Additional Options [Member]
Shares registered and available for issuance
Shares registered	55,514
Employee Stock Purchase Plan [Member]
Shares registered and available for issuance
Shares registered			4,696,424
Towers Watson Long Term Incentive Plan [Member]
Shares registered and available for issuance
Shares registered			12,500,000
Watson Wyatt Long Term Incentive Plan [Member]
Shares registered and available for issuance
Shares registered			125,648