Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Commissions									$ 2,338.7	$ 2,083.0	$ 1,553.1
Fees									1,432.3	1,258.3	1,059.5
Supplemental commissions									125.5	104.0	77.3
Contingent commissions									93.7	84.7	52.1
Investment income									54.2	41.3	8.1
Gains on books of business sales and other									6.7	7.3	5.2
Revenue from clean coal activities									1,310.8	1,029.5	412.5
Other - net gain									30.5	18.4	11.8
Total revenues	$ 1,334.9	$ 1,454.8	$ 1,371.4	$ 1,231.3	$ 1,245.4	$ 1,286.8	$ 1,179.3	$ 915.0	5,392.4	4,626.5	3,179.6
Compensation									2,428.9	2,167.6	1,685.0
Operating									840.7	743.1	535.9
Cost of revenues from clean coal activities									1,351.5	1,058.9	437.3
Interest									103.0	89.0	50.1
Depreciation									93.9	69.4	53.4
Amortization									240.3	189.5	125.2
Change in estimated acquisition earnout payables									40.6	17.5	1.7
Total expenses									5,098.9	4,335.0	2,888.6
Earnings (loss) before income taxes									293.5	291.5	291.0
Provision (benefit) for income taxes									(95.6)	(36.0)	6.4
Net earnings									389.1	327.5	284.6
Net earnings (loss) attributable to controlling interests									32.3	24.1	16.0
Net earnings attributable to controlling interests	62.3	133.3	139.3	21.9	51.5	93.6	109.0	49.3	356.8	303.4	268.6
Net foreign exchange gain (loss)									0.2	0.5	0.2
Total revenues	1,334.9	$ 1,454.8	$ 1,371.4	$ 1,231.3	1,245.4	$ 1,286.8	$ 1,179.3	$ 915.0	5,392.4	4,626.5	3,179.6
Total assets	10,913.8				10,010.0				10,913.8	10,010.0	6,860.5
Goodwill - net	3,662.9				3,449.6				3,662.9	3,449.6	2,145.2
Amortizable intangible assets - net	1,698.8				1,776.0				1,698.8	1,776.0	1,078.8
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3,635.6				3,427.5				3,635.6	3,427.5	2,122.9
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	27.3				22.1				27.3	22.1	22.3
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0.0				0.0				0.0	0.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									4,041.4	3,442.8	2,533.5
Total revenues									4,041.4	3,442.8	2,533.5
Total assets	5,882.9				5,046.6				5,882.9	5,046.6	4,422.4
Goodwill - net	1,970.4				1,672.8				1,970.4	1,672.8
Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	1,946.9				1,652.6				1,946.9	1,652.6
Unites States [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	23.5				20.2				23.5	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									766.9	726.2	427.9
Total revenues									766.9	726.2	427.9
Total assets	2,652.1				2,450.4				2,652.1	2,450.4	1,878.3
Goodwill - net	782.8				820.6				782.8	820.6
United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	779.3				818.7				779.3	818.7
United Kingdom [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3.5				1.9				3.5	1.9
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									256.7	236.6	152.6
Total revenues									256.7	236.6	152.6
Total assets	951.4				1,031.2				951.4	1,031.2	277.9
Goodwill - net	380.1				336.8				380.1	336.8
Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	380.1				336.8				380.1	336.8
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									136.6	85.0	32.6
Total revenues									136.6	85.0	32.6
Total assets	578.1				642.0				578.1	642.0	108.8
Goodwill - net	282.6				318.5				282.6	318.5
Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	282.6				318.5				282.6	318.5
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
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									122.6	81.3	1.5
Total revenues									122.6	81.3	1.5
Total assets	627.2				615.7				627.2	615.7	1.8
Goodwill - net	204.5				258.7				204.5	258.7
New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	204.2				258.7				204.2	258.7
New Zealand [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0.3								0.3
New Zealand [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0.0								0.0
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									68.2	54.6	31.5
Total revenues									68.2	54.6	31.5
Total assets	222.1				224.1				222.1	224.1	171.3
Goodwill - net	42.5				42.2				42.5	42.2
Other Foreign [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	42.5				42.2				42.5	42.2
Other Foreign [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0.0								0.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Commissions									2,338.7	2,083.0	1,553.1
Fees									705.8	577.0	432.5
Supplemental commissions									125.5	104.0	77.3
Contingent commissions									93.7	84.7	52.1
Investment income									53.6	40.3	6.1
Gains on books of business sales and other									6.7	7.3	5.2
Total revenues									3,324.0	2,896.3	2,126.3
Compensation									1,939.7	1,703.1	1,277.9
Operating									638.1	530.1	364.7
Depreciation									54.4	44.4	30.8
Amortization									237.3	186.3	122.3
Change in estimated acquisition earnout payables									41.1	17.6	3.4
Total expenses									2,910.6	2,481.5	1,799.1
Earnings (loss) before income taxes									413.4	414.8	327.2
Provision (benefit) for income taxes									145.3	151.0	122.2
Net earnings									268.1	263.8	205.0
Net earnings (loss) attributable to controlling interests									1.7	0.9	1.7
Net earnings attributable to controlling interests									266.4	262.9	203.3
Net foreign exchange gain (loss)									(0.2)	1.1	0.6
Total revenues									3,324.0	2,896.3	2,126.3
Total assets	8,969.7				8,386.2				8,969.7	8,386.2	5,494.6
Goodwill - net	3,635.6				3,427.5				3,635.6	3,427.5	2,122.9
Amortizable intangible assets - net	1,677.8				1,757.3				1,677.8	1,757.3	1,057.4
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Fees									726.5	681.3	627.0
Investment income									0.6	1.0	2.0
Total revenues									727.1	682.3	629.0
Compensation									427.2	414.2	383.0
Operating									180.8	176.4	149.0
Depreciation									24.3	21.2	19.7
Amortization									3.0	3.2	2.9
Change in estimated acquisition earnout payables									(0.5)	(0.1)	(1.7)
Total expenses									634.8	614.9	552.9
Earnings (loss) before income taxes									92.3	67.4	76.1
Provision (benefit) for income taxes									35.1	25.3	28.4
Net earnings									57.2	42.1	47.7
Net earnings attributable to controlling interests									57.2	42.1	47.7
Total revenues									727.1	682.3	629.0
Total assets	660.1				574.9				660.1	574.9	572.8
Goodwill - net	27.3				22.1				27.3	22.1	22.3
Amortizable intangible assets - net	21.0				18.7				21.0	18.7	21.4
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenue from clean coal activities									1,310.8	1,029.5	412.5
Other - net gain									30.5	18.4	11.8
Total revenues									1,341.3	1,047.9	424.3
Compensation									62.0	50.3	24.1
Operating									21.8	36.6	22.2
Cost of revenues from clean coal activities									1,351.5	1,058.9	437.3
Interest									103.0	89.0	50.1
Depreciation									15.2	3.8	2.9
Total expenses									1,553.5	1,238.6	536.6
Earnings (loss) before income taxes									(212.2)	(190.7)	(112.3)
Provision (benefit) for income taxes									(276.0)	(212.3)	(144.2)
Net earnings									63.8	21.6	31.9
Net earnings (loss) attributable to controlling interests									30.6	23.2	14.3
Net earnings attributable to controlling interests									33.2	(1.6)	17.6
Net foreign exchange gain (loss)									0.4	(0.6)	(0.4)
Total revenues									1,341.3	1,047.9	424.3
Total assets	1,284.0				1,048.9				1,284.0	1,048.9	793.1
Operating Segments [Member] | Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									2,122.1	1,873.3	1,626.8
Total revenues									2,122.1	1,873.3	1,626.8
Total assets	4,092.8				3,557.1				4,092.8	3,557.1	3,191.5
Operating Segments [Member] | Unites States [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									591.8	532.6	491.5
Total revenues									591.8	532.6	491.5
Total assets	525.2				457.5				525.2	457.5	447.1
Operating Segments [Member] | Unites States [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									1,327.5	1,036.9	415.2
Total revenues									1,327.5	1,036.9	415.2
Total assets	1,264.9				1,032.0				1,264.9	1,032.0	783.8
Operating Segments [Member] | United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									738.5	696.8	400.5
Total revenues									738.5	696.8	400.5
Total assets	2,580.0				2,376.4				2,580.0	2,376.4	1,819.5
Operating Segments [Member] | United Kingdom [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									28.4	29.4	27.4
Total revenues									28.4	29.4	27.4
Total assets	72.1				74.0				72.1	74.0	58.8
Operating Segments [Member] | Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									157.3	122.4	47.1
Total revenues									157.3	122.4	47.1
Total assets	895.8				992.2				895.8	992.2	214.3
Operating Segments [Member] | Australia [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									99.4	114.2	105.5
Total revenues									99.4	114.2	105.5
Total assets	55.6				39.0				55.6	39.0	63.6
Operating Segments [Member] | Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									133.1	81.8	29.5
Total revenues									133.1	81.8	29.5
Total assets	575.0				639.2				575.0	639.2	107.3
Operating Segments [Member] | Canada [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									3.5	3.2	3.1
Total revenues									3.5	3.2	3.1
Total assets	3.1				2.8				3.1	2.8	1.5
Operating Segments [Member] | New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									118.6	78.4
Total revenues									118.6	78.4
Total assets	623.1				614.1				623.1	614.1
Operating Segments [Member] | New Zealand [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									4.0	2.9	1.5
Total revenues									4.0	2.9	1.5
Total assets	4.1				1.6				4.1	1.6	1.8
Operating Segments [Member] | Other Foreign [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									54.4	43.6	22.4
Total revenues									54.4	43.6	22.4
Total assets	203.0				207.2				203.0	207.2	162.0
Operating Segments [Member] | Other Foreign [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									13.8	11.0	9.1
Total revenues									13.8	11.0	9.1
Total assets	$ 19.1				$ 16.9				$ 19.1	$ 16.9	$ 9.3