Commitments, Contingencies and off-Balance Sheet Arrangements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended			132 Months Ended
	Dec. 31, 2012 Acquisition sqft	Dec. 31, 2011 Acquisition	Dec. 31, 2010 Acquisition	Dec. 31, 2012 Entity
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Acquired property through lease area, sqft.	306,000
Acquired property through lease percentage of building	60.00%
Operating lease commitment, expiration date	Feb. 28, 2018
Total rent expense	$ 91.0	$ 91.0	$ 81.5
Number of companies acquired	46	22	11	248
Aggregate amount of maximum earnout obligations related to acquisitions	384.8
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet	139.8
Liabilities recorded on self-insurance	10.2			10.2
Debt	32.4			32.4
Income tax credits and adjustments	130.0
Maximum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	1.00%			1.00%
Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	50.00%			50.00%
Tax-Advantaged Investments No Longer Held [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Total indemnifications	160.0			160.0
Self-Insurance Deductibles [Member] | Letter of Credit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	8.5			8.5
Rent-A-Captive Facility [Member] | Letter of Credit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	5.7			5.7
Errors and Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained	5.0
Amount of losses in excess of retained amounts	175.0			175.0
Errors and Omissions [Member] | Maximum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	1.7			1.7
Errors and Omissions [Member] | Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	$ 4.7			$ 4.7