Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Commissions									$ 1,302.5	$ 1,127.4	$ 957.3
Fees									971.7	870.2	735.0
Supplemental commissions									67.9	56.0	60.8
Contingent commissions									42.9	38.1	36.8
Investment income									10.4	8.1	6.9
Gains on books of business sales and other									3.9	5.5	5.9
Revenue from clean coal activities									119.6	29.2	65.6
Other - net gain									1.4	0.2	(4.1)
Total revenues	673.2	650.4	649.9	546.8	578.4	562.8	546.1	447.4	2,520.3	2,134.7	1,864.2
Compensation									1,493.4	1,326.1	1,117.5
Operating									483.2	419.0	354.6
Cost of revenues from clean coal activities									111.6	32.0	64.0
Interest									43.0	40.8	34.6
Depreciation									41.4	35.9	32.3
Amortization									99.0	79.3	60.8
Change in estimated acquisition earnout payables									3.4	(6.2)	(2.6)
Total expenses	627.8	574.1	558.8	514.3	531.0	491.8	480.2	423.9	2,275.0	1,926.9	1,661.2
Earnings (loss) from continuing operations before income taxes									245.3	207.8	203.0
Provision (benefit) for income taxes									50.3	63.7	39.7
Earnings (loss) from continuing operations	33.5	61.7	71.7	28.1	40.5	46.7	41.7	15.2	195.0	144.1	163.3
Net foreign exchange gain (loss)									(1.9)	(0.2)	(0.8)
Total revenues	673.2	650.4	649.9	546.8	578.4	562.8	546.1	447.4	2,520.3	2,134.7	1,864.2
Total assets	5,352.3				4,483.5				5,352.3	4,483.5	3,596.0
Goodwill - net	1,472.7				1,155.3				1,472.7	1,155.3	883.7
Amortizable intangible assets - net	809.6				561.5				809.6	561.5	483.5
United States [Member]
Segment Reporting Information [Line Items]
Total revenues									2,006.1	1,725.1	1,613.5
Total revenues									2,006.1	1,725.1	1,613.5
Total assets	3,675.9				3,202.5				3,675.9	3,202.5	2,895.5
Goodwill - net	1,177.3				969.5				1,177.3	969.5
Foreign, principally Australia, Canada and the U.K. [Member]
Segment Reporting Information [Line Items]
Total revenues									514.2	409.6	250.7
Total revenues									514.2	409.6	250.7
Total assets	1,676.4				1,281.0				1,676.4	1,281.0	700.5
Goodwill - net	295.4				185.8				295.4	185.8
Brokerage [Member]
Segment Reporting Information [Line Items]
Commissions									1,302.5	1,127.4	957.3
Fees									403.2	324.1	274.9
Supplemental commissions									67.9	56.0	60.8
Contingent commissions									42.9	38.1	36.8
Investment income									7.2	5.4	4.9
Gains on books of business sales and other									3.9	5.5	5.9
Total revenues									1,827.6	1,556.5	1,340.6
Compensation									1,131.6	968.4	817.1
Operating									312.7	267.3	223.6
Depreciation									24.7	21.2	19.5
Amortization									96.2	77.0	59.8
Change in estimated acquisition earnout payables									3.6	(6.2)	(2.6)
Total expenses									1,568.8	1,327.7	1,117.4
Earnings (loss) from continuing operations before income taxes									258.8	228.8	223.2
Provision (benefit) for income taxes									103.0	88.6	87.7
Earnings (loss) from continuing operations									155.8	140.2	135.5
Net foreign exchange gain (loss)									(1.6)		(0.9)
Total revenues									1,827.6	1,556.5	1,340.6
Total assets	4,196.8				3,346.6				4,196.8	3,346.6	2,560.7
Goodwill - net	1,451.4				1,136.6				1,451.4	1,136.6	866.6
Amortizable intangible assets - net	791.6				542.3				791.6	542.3	461.9
Brokerage [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total revenues									1,431.6	1,266.2	1,169.6
Total revenues									1,431.6	1,266.2	1,169.6
Total assets	2,637.1				2,215.3				2,637.1	2,215.3	1,995.8
Goodwill - net	1,158.1				951.0				1,158.1	951.0
Brokerage [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Segment Reporting Information [Line Items]
Total revenues									396.0	290.3	171.0
Total revenues									396.0	290.3	171.0
Total assets	1,559.7				1,131.3				1,559.7	1,131.3	564.9
Goodwill - net	293.3				185.6				293.3	185.6
Risk Management [Member]
Segment Reporting Information [Line Items]
Fees									568.5	546.1	460.1
Investment income									3.2	2.7	2.0
Total revenues									571.7	548.8	462.1
Compensation									347.0	344.1	288.0
Operating									137.7	135.8	109.1
Depreciation									16.0	14.2	12.4
Amortization									2.8	2.3	1.0
Change in estimated acquisition earnout payables									(0.2)
Total expenses									503.3	496.4	410.5
Earnings (loss) from continuing operations before income taxes									68.4	52.4	51.6
Provision (benefit) for income taxes									25.9	19.1	20.3
Earnings (loss) from continuing operations									42.5	33.3	31.3
Net foreign exchange gain (loss)									(0.1)		0.1
Total revenues									571.7	548.8	462.1
Total assets	498.6				529.1				498.6	529.1	521.3
Goodwill - net	21.3				18.7				21.3	18.7	17.1
Amortizable intangible assets - net	18.0				19.2				18.0	19.2	21.6
Risk Management [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total revenues									453.5	429.5	382.0
Total revenues									453.5	429.5	382.0
Total assets	390.9				384.5				390.9	384.5	388.6
Goodwill - net	19.2				18.5				19.2	18.5
Risk Management [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Segment Reporting Information [Line Items]
Total revenues									118.2	119.3	80.1
Total revenues									118.2	119.3	80.1
Total assets	107.7				144.6				107.7	144.6	132.7
Goodwill - net	2.1				0.2				2.1	0.2
Corporate [Member]
Segment Reporting Information [Line Items]
Revenue from clean coal activities									119.6	29.2	65.6
Other - net gain									1.4	0.2	(4.1)
Total revenues									121.0	29.4	61.5
Compensation									14.8	13.6	12.4
Operating									32.8	15.9	21.9
Cost of revenues from clean coal activities									111.6	32.0	64.0
Interest									43.0	40.8	34.6
Depreciation									0.7	0.5	0.4
Total expenses									202.9	102.8	133.3
Earnings (loss) from continuing operations before income taxes									(81.9)	(73.4)	(71.8)
Provision (benefit) for income taxes									(78.6)	(44.0)	(68.3)
Earnings (loss) from continuing operations									(3.3)	(29.4)	(3.5)
Net foreign exchange gain (loss)									(0.2)	(0.2)
Total revenues									121.0	29.4	61.5
Total assets	656.9				607.8				656.9	607.8	514.0
Goodwill - net	0				0				0	0
Corporate [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total revenues									121.0	29.4	61.9
Total revenues									121.0	29.4	61.9
Total assets	647.9				602.7				647.9	602.7	511.1
Goodwill - net	0				0				0	0
Corporate [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Segment Reporting Information [Line Items]
Total revenues											(0.4)
Total revenues											(0.4)
Total assets	9.0				5.1				9.0	5.1	2.9
Goodwill - net	$ 0				$ 0				$ 0	$ 0