Mergers and Acquisitions (Details) (USD $)	12 Months Ended					3 Months Ended			3 Months Ended							12 Months Ended				12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Extend Health [Member]	Jun. 30, 2012 Extend Health [Member] Customer Related Intangibles [Member]	Jun. 30, 2012 Extend Health [Member] Developed Technology Rights [Member]	Jun. 30, 2012 Extend Health [Member] Other Intangibles [Member]	Dec. 31, 2010 Aliquant [Member]	Dec. 31, 2010 Aliquant [Member] Customer Related Intangibles [Member]	Dec. 31, 2010 Aliquant [Member] Developed Technology Rights [Member]	Mar. 31, 2011 EMB [Member]	Mar. 31, 2011 EMB [Member] Common Class B-3 [Member]	Mar. 31, 2011 EMB [Member] Common Class B-4 [Member]	Mar. 31, 2011 EMB [Member] Customer Related Intangibles [Member]	Mar. 31, 2011 EMB [Member] Developed Technology Rights [Member]	Mar. 31, 2011 EMB [Member] In Process Technology [Member]	Mar. 31, 2011 EMB [Member] Above Market Leases [Member]	Jun. 30, 2010 Towers Perrin Merger [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Watson Wyatt [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Towers Perrin [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Trade Names [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Common Class A [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Restricted Class A [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Common Class B [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Common Class R [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Customer Related Intangibles [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] External Use Developed Technology Rights [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Internal Use Developed Technology Rights [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Below Market Leases [Member]	Jun. 30, 2010 Towers Perrin Merger [Member] Above Market Leases [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity		Extend Health				Aliquant			EMB							Towers Perrin Merger
Business Acquisition, Effective Date of Acquisition		2012-05-29				2010-12-31			2011-01-31							2010-01-01
Business Acquisition, Cost of Acquired Entity, Cash Paid		$ 435,000,000				$ 67,700,000			$ 69,800,000							$ 200,000,000
Business Combination, Consideration Transferred		446,200,000							101,900,000							1,800,000,000
Stock options assumed in Extend Health acquisition	11,160,000	11,200,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned									11,400,000
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares										113,858	113,858									46,911,275	4,248,984	29,483,008	8,548,835
Business Acquisition, Notes to shareholders																200,000,000
Business Acquisition, Contingent Consideration, Potential Cash Payment									16,900,000
Business Acquisition Expected Cost Of Compensation									27,900,000
Business Acquisition, Purchase Price Allocation, Amortizable Intangible Assets			123,200,000	26,700,000	1,000,000		13,900,000	4,000,000				13,500,000	12,100,000	1,600,000	600,000									140,800,000	58,200,000	67,200,000	11,100,000	28,600,000
Acquired Indefinite-lived Intangible Asset, Amount																			275,500,000
Business Acquisition, Current Receivable Acquired		3,100,000
Business Acquisition, Purchase Price Allocation, Deferred Income Taxes, Asset (Liability), Net		53,800,000							2,900,000							149,400,000
Business Acquisition Deferred Maintenance Adjustment		14,600,000							9,600,000
Business Acquisition, Purchase Price Allocation, Goodwill Amount		$ 341,000,000				$ 49,500,000			$ 59,000,000							$ 1,300,000,000
Business Acquisition, Preexisting Relationship [Abstract]
Business Combination Step Acquisition Pre Existing Equity Interest In Acquiree Percentage																	36.40%	36.40%