Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Commissions									$ 2,083.0	$ 1,553.1	$ 1,302.5
Fees									1,258.3	1,059.5	971.7
Supplemental commissions									104.0	77.3	67.9
Contingent commissions									84.7	52.1	42.9
Investment income									41.3	8.1	10.4
Gains on books of business sales and other									7.3	5.2	3.9
Revenue from clean coal activities									1,029.5	412.5	119.6
Other - net gain									18.4	11.8	1.4
Total revenues	1,245.4	1,286.8	1,179.3	915.0	890.2	835.8	779.5	674.1	4,626.5	3,179.6	2,520.3
Compensation									2,167.6	1,685.0	1,493.4
Operating									767.2	552.4	483.2
Cost of revenues from clean coal activities									1,058.9	437.3	111.6
Interest									89.0	50.1	43.0
Depreciation									69.4	53.4	41.4
Amortization									189.5	125.2	99.0
Change in estimated acquisition earnout payables									17.5	1.7	3.4
Total expenses	(1,219.1)	(1,199.1)	(1,072.2)	(868.7)	(840.9)	(750.3)	(682.1)	(631.8)	4,359.1	2,905.1	2,275.0
Earnings (loss) before income taxes	26.3	87.7	107.1	46.3	49.3	85.5	97.4	42.3	267.4	274.5	245.3
Provision (benefit) for income taxes									(36.0)	5.9	50.3
Net earnings (loss)									303.4	268.6	195.0
Net foreign exchange gain (loss)									0.5	0.2	(1.9)
Total revenues	1,245.4	1,286.8	1,179.3	915.0	890.2	835.8	779.5	674.1	4,626.5	3,179.6	2,520.3
Total assets	10,010.0				6,860.5				10,010.0	6,860.5	5,352.3
Goodwill - net	3,449.6				2,145.2				3,449.6	2,145.2	1,472.7
Amortizable intangible assets - net	1,776.0				1,078.8				1,776.0	1,078.8	809.6
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3,427.5				2,122.9				3,427.5	2,122.9	1,451.4
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	22.1				22.3				22.1	22.3	21.3
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0	0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									3,454.9	2,542.6	2,006.1
Total revenues									3,454.9	2,542.6	2,006.1
Total assets	5,046.6				4,422.4				5,046.6	4,422.4	3,675.9
Goodwill - net	1,672.8				1,469.8				1,672.8	1,469.8
Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	1,652.6				1,449.6				1,652.6	1,449.6
Unites States [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	20.2				20.2				20.2	20.2
Unites States [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									726.4	427.9	346.0
Total revenues									726.4	427.9	346.0
Total assets					1,878.3					1,878.3	1,170.0
Goodwill - net	820.6				584.9				820.6	584.9
United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	818.7				582.8				818.7	582.8
United Kingdom [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	1.9				2.1				1.9	2.1
United Kingdom [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									243.1	152.6	121.4
Total revenues									243.1	152.6	121.4
Total assets	642.0				277.9				642.0	277.9	260.5
Goodwill - net	336.8				37.1				336.8	37.1
Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	336.8				37.1				336.8	37.1
Australia [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Australia [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									85.0	32.6	32.1
Total revenues									85.0	32.6	32.1
Total assets	1,031.2				108.8				1,031.2	108.8	102.4
Goodwill - net	318.5				26.8				318.5	26.8
Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	318.5				26.8				318.5	26.8
Canada [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Canada [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Other Foreign, Principally New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									117.1
Total revenues									117.1
Total assets	839.8								839.8
Goodwill - net	300.9				26.6				300.9	26.6
Other Foreign, Principally New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	300.9				26.6				300.9	26.6
Other Foreign, Principally New Zealand [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Other Foreign, Principally New Zealand [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0				0				0	0
Other Foreign, Principally Bermuda [Member]
Segment Reporting Information [Line Items]
Total revenues										23.9	14.7
Total revenues										23.9	14.7
Total assets					173.1					173.1	143.5
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Commissions									2,083.0	1,553.1	1,302.5
Fees									595.0	450.5	403.2
Supplemental commissions									104.0	77.3	67.9
Contingent commissions									84.7	52.1	42.9
Investment income									40.3	6.1	7.2
Gains on books of business sales and other									7.3	5.2	3.9
Total revenues									2,914.3	2,144.3	1,827.6
Compensation									1,715.7	1,290.4	1,131.6
Operating									534.1	369.9	312.7
Depreciation									44.7	31.1	24.7
Amortization									186.7	122.7	96.2
Change in estimated acquisition earnout payables									17.5	2.6	3.6
Total expenses									2,498.7	1,816.7	1,568.8
Earnings (loss) before income taxes									415.6	327.6	258.8
Provision (benefit) for income taxes									151.8	122.8	103.0
Net earnings (loss)									263.8	204.8	155.8
Net foreign exchange gain (loss)									1.1	0.6	(1.6)
Total revenues									2,914.3	2,144.3	1,827.6
Total assets	8,413.4				5,522.7				8,413.4	5,522.7	4,196.8
Goodwill - net	3,427.5				2,122.9				3,427.5	2,122.9	1,451.4
Amortizable intangible assets - net	1,761.2				1,061.6				1,761.2	1,061.6	791.6
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Fees									663.3	609.0	568.5
Investment income									1.0	2.0	3.2
Total revenues									664.3	611.0	571.7
Compensation									401.6	370.5	347.0
Operating									173.3	146.0	137.7
Depreciation									20.9	19.4	16.0
Amortization									2.8	2.5	2.8
Change in estimated acquisition earnout payables										(0.9)	(0.2)
Total expenses									598.6	537.5	503.3
Earnings (loss) before income taxes									65.7	73.5	68.4
Provision (benefit) for income taxes									24.5	27.3	25.9
Net earnings (loss)									41.2	46.2	42.5
Net foreign exchange gain (loss)											(0.1)
Total revenues									664.3	611.0	571.7
Total assets	547.7				544.7				547.7	544.7	498.6
Goodwill - net	22.1				22.3				22.1	22.3	21.3
Amortizable intangible assets - net	14.8				17.2				14.8	17.2	18.0
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenue from clean coal activities									1,029.5	412.5	119.6
Other - net gain									18.4	11.8	1.4
Total revenues									1,047.9	424.3	121.0
Compensation									50.3	24.1	14.8
Operating									59.8	36.5	32.8
Cost of revenues from clean coal activities									1,058.9	437.3	111.6
Interest									89.0	50.1	43.0
Depreciation									3.8	2.9	0.7
Total expenses									1,261.8	550.9	202.9
Earnings (loss) before income taxes									(213.9)	(126.6)	(81.9)
Provision (benefit) for income taxes									(212.3)	(144.2)	(78.6)
Net earnings (loss)									(1.6)	17.6	(3.3)
Net foreign exchange gain (loss)									(0.6)	(0.4)	(0.2)
Total revenues									1,047.9	424.3	121.0
Total assets	1,048.9				793.1				1,048.9	793.1	656.9
Operating Segments [Member] | Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									1,891.3	1,644.8	1,431.6
Total revenues									1,891.3	1,644.8	1,431.6
Total assets	3,584.3				3,219.6				3,584.3	3,219.6	2,637.1
Operating Segments [Member] | Unites States [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									514.7	473.5	453.5
Total revenues									514.7	473.5	453.5
Total assets	430.3				419.0				430.3	419.0	390.9
Operating Segments [Member] | Unites States [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									1,048.9	424.3	121.0
Total revenues									1,048.9	424.3	121.0
Total assets	1,032.0				783.8				1,032.0	783.8	647.9
Operating Segments [Member] | United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									697.1	400.5	317.8
Total revenues									697.1	400.5	317.8
Total assets	2,376.4				1,819.5				2,376.4	1,819.5	1,117.6
Operating Segments [Member] | United Kingdom [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									29.3	27.4	28.2
Total revenues									29.3	27.4	28.2
Total assets	74.0				58.8				74.0	58.8	52.4
Operating Segments [Member] | Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									128.9	47.1	35.1
Total revenues									128.9	47.1	35.1
Total assets	639.2				214.3				639.2	214.3	208.4
Operating Segments [Member] | Australia [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									114.2	105.5	86.3
Total revenues									114.2	105.5	86.3
Total assets	2.8				63.6				2.8	63.6	52.1
Operating Segments [Member] | Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									81.8	29.5	28.9
Total revenues									81.8	29.5	28.9
Total assets	992.2				107.3				992.2	107.3	100.7
Operating Segments [Member] | Canada [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									3.2	3.1	3.2
Total revenues									3.2	3.1	3.2
Total assets	39.0				1.5				39.0	1.5	1.7
Operating Segments [Member] | Other Foreign, Principally New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									115.2
Total revenues									115.2
Total assets	821.3								821.3
Operating Segments [Member] | Other Foreign, Principally New Zealand [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									2.9
Total revenues									2.9
Total assets	1.6								1.6
Operating Segments [Member] | Other Foreign, Principally New Zealand [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									(1.0)
Total revenues									(1.0)
Total assets	16.9								16.9
Operating Segments [Member] | Other Foreign, Principally Bermuda [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										22.4	14.2
Total revenues										22.4	14.2
Total assets					162.0					162.0	133.0
Operating Segments [Member] | Other Foreign, Principally Bermuda [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues										1.5	0.5
Total revenues										1.5	0.5
Total assets					1.8					1.8	1.5
Operating Segments [Member] | Other Foreign, Principally Bermuda [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total assets					$ 9.3					$ 9.3	$ 9.0