Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Commissions									$ 2,439.1	$ 2,338.7	$ 2,083.0
Fees									1,492.8	1,432.3	1,258.3
Supplemental commissions									147.0	125.5	104.0
Contingent commissions									107.2	93.7	84.7
Investment income									53.3	54.2	41.3
Gains on books of business sales and other									6.6	6.7	7.3
Revenue from clean coal activities									1,350.1	1,310.8	1,029.5
Other - net gain									(1.3)	30.5	18.4
Total revenues	$ 1,385.0	$ 1,482.3	$ 1,427.1	$ 1,300.4	$ 1,334.9	$ 1,454.8	$ 1,371.4	$ 1,231.3	5,594.8	5,392.4	4,626.5
Compensation									2,538.9	2,428.9	2,167.6
Operating									797.7	840.7	743.1
Cost of revenues from clean coal activities									1,408.6	1,351.5	1,058.9
Interest									109.8	103.0	89.0
Depreciation									103.6	93.9	69.4
Amortization									247.2	240.3	189.5
Change in estimated acquisition earnout payables									32.1	40.6	17.5
Total expenses									5,237.9	5,098.9	4,335.0
Earnings (loss) before income taxes									356.9	293.5	291.5
Provision (benefit) for income taxes									(88.1)	(95.6)	(36.0)
Net earnings									445.0	389.1	327.5
Net earnings attributable to noncontrolling interests									30.6	32.3	24.1
Net earnings (loss) attributable to controlling interests	95.1	122.8	150.0	46.5	62.3	133.3	139.3	21.9	414.4	356.8	303.4
Net foreign exchange gain (loss)									3.0	0.2	0.5
Total revenues	1,385.0	$ 1,482.3	$ 1,427.1	$ 1,300.4	1,334.9	$ 1,454.8	$ 1,371.4	$ 1,231.3	5,594.8	5,392.4	4,626.5
Total assets	11,489.6				10,910.5				11,489.6	10,910.5	10,010.0
Goodwill - net	3,767.8				3,662.9				3,767.8	3,662.9	3,449.6
Amortizable intangible assets - net	1,627.3				1,698.8				1,627.3	1,698.8	1,776.0
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3,736.9				3,635.6				3,736.9	3,635.6	3,427.5
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.8								2.8
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									4,272.6	4,041.4	3,442.8
Total revenues									4,272.6	4,041.4	3,442.8
Total assets	6,556.3				5,882.9				6,556.3	5,882.9	5,046.6
Goodwill - net	2,138.5				1,970.4				2,138.5	1,970.4
Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2,115.0				1,946.9				2,115.0	1,946.9
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									712.1	766.9	726.2
Total revenues									712.1	766.9	726.2
Total assets	2,383.7				2,652.1				2,383.7	2,652.1	2,450.4
Goodwill - net	666.5				782.8				666.5	782.8
United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	662.2				779.3				662.2	779.3
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									245.5	256.7	236.6
Total revenues									245.5	256.7	236.6
Total assets	951.3				951.4				951.3	951.4	1,031.2
Goodwill - net	382.7				380.1				382.7	380.1
Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	382.7				380.1				382.7	380.1
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									138.2	136.6	85.0
Total revenues									138.2	136.6	85.0
Total assets	576.1				578.1				576.1	578.1	642.0
Goodwill - net	292.2				282.6				292.2	282.6
Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	292.2				282.6				292.2	282.6
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									125.8	122.6	81.3
Total revenues									125.8	122.6	81.3
Total assets	673.3				627.2				673.3	627.2	615.7
Goodwill - net	205.3				204.5				205.3	204.5
New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	205.0				204.2				205.0	204.2
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									100.6	68.2	54.6
Total revenues									100.6	68.2	54.6
Total assets	348.9				222.1				348.9	222.1	224.1
Goodwill - net	82.6				42.5				82.6	42.5
Other Foreign [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	79.8				42.5				79.8	42.5
Other Foreign [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.8								2.8
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Commissions									2,439.1	2,338.7	2,083.0
Fees									775.7	705.8	577.0
Supplemental commissions									147.0	125.5	104.0
Contingent commissions									107.2	93.7	84.7
Investment income									52.3	53.6	40.3
Gains on books of business sales and other									6.6	6.7	7.3
Total revenues									3,527.9	3,324.0	2,896.3
Compensation									2,041.8	1,939.7	1,703.1
Operating									600.9	638.1	530.1
Depreciation									57.2	54.4	44.4
Amortization									244.7	237.3	186.3
Change in estimated acquisition earnout payables									32.1	41.1	17.6
Total expenses									2,976.7	2,910.6	2,481.5
Earnings (loss) before income taxes									551.2	413.4	414.8
Provision (benefit) for income taxes									194.1	145.3	151.0
Net earnings									357.1	268.1	263.8
Net earnings attributable to noncontrolling interests									3.6	1.7	0.9
Net earnings (loss) attributable to controlling interests									353.5	266.4	262.9
Net foreign exchange gain (loss)									2.9	(0.2)	1.1
Total revenues									3,527.9	3,324.0	2,896.3
Total assets	9,183.4				8,969.7				9,183.4	8,969.7	8,386.2
Goodwill - net	3,736.9				3,635.6				3,736.9	3,635.6	3,427.5
Amortizable intangible assets - net	1,613.6				1,677.8				1,613.6	1,677.8	1,757.3
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Fees									717.1	726.5	681.3
Investment income									1.0	0.6	1.0
Total revenues									718.1	727.1	682.3
Compensation									424.5	427.2	414.2
Operating									171.4	180.8	176.4
Depreciation									27.2	24.3	21.2
Amortization									2.5	3.0	3.2
Change in estimated acquisition earnout payables										(0.5)	(0.1)
Total expenses									625.6	634.8	614.9
Earnings (loss) before income taxes									92.5	92.3	67.4
Provision (benefit) for income taxes									35.3	35.1	25.3
Net earnings									57.2	57.2	42.1
Net earnings (loss) attributable to controlling interests									57.2	57.2	42.1
Total revenues									718.1	727.1	682.3
Total assets	666.4				660.1				666.4	660.1	574.9
Goodwill - net	28.1				27.3				28.1	27.3	22.1
Amortizable intangible assets - net	13.7				21.0				13.7	21.0	18.7
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenue from clean coal activities									1,350.1	1,310.8	1,029.5
Other - net gain									(1.3)	30.5	18.4
Total revenues									1,348.8	1,341.3	1,047.9
Compensation									72.6	62.0	50.3
Operating									25.4	21.8	36.6
Cost of revenues from clean coal activities									1,408.6	1,351.5	1,058.9
Interest									109.8	103.0	89.0
Depreciation									19.2	15.2	3.8
Total expenses									1,635.6	1,553.5	1,238.6
Earnings (loss) before income taxes									(286.8)	(212.2)	(190.7)
Provision (benefit) for income taxes									(317.5)	(276.0)	(212.3)
Net earnings									30.7	63.8	21.6
Net earnings attributable to noncontrolling interests									27.0	30.6	23.2
Net earnings (loss) attributable to controlling interests									3.7	33.2	(1.6)
Net foreign exchange gain (loss)									0.1	0.4	(0.6)
Total revenues									1,348.8	1,341.3	1,047.9
Total assets	1,639.8				1,284.0				1,639.8	1,284.0	1,048.9
Goodwill - net	2.8								2.8
Operating Segments [Member] | Unites States [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									2,334.4	2,122.1	1,873.3
Total revenues									2,334.4	2,122.1	1,873.3
Total assets	4,393.6				4,092.8				4,393.6	4,092.8	3,557.1
Operating Segments [Member] | Unites States [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									610.3	591.8	532.6
Total revenues									610.3	591.8	532.6
Total assets	540.5				525.2				540.5	525.2	457.5
Operating Segments [Member] | Unites States [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									1,327.9	1,327.5	1,036.9
Total revenues									1,327.9	1,327.5	1,036.9
Total assets	1,622.2				1,264.9				1,622.2	1,264.9	1,032.0
Operating Segments [Member] | United Kingdom [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									686.5	738.5	696.8
Total revenues									686.5	738.5	696.8
Total assets	2,321.9				2,580.0				2,321.9	2,580.0	2,376.4
Operating Segments [Member] | United Kingdom [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									25.6	28.4	29.4
Total revenues									25.6	28.4	29.4
Total assets	61.8				72.1				61.8	72.1	74.0
Operating Segments [Member] | Australia [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									172.5	157.3	122.4
Total revenues									172.5	157.3	122.4
Total assets	894.4				895.8				894.4	895.8	992.2
Operating Segments [Member] | Australia [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									73.0	99.4	114.2
Total revenues									73.0	99.4	114.2
Total assets	56.9				55.6				56.9	55.6	39.0
Operating Segments [Member] | Canada [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									134.1	133.1	81.8
Total revenues									134.1	133.1	81.8
Total assets	573.3				575.0				573.3	575.0	639.2
Operating Segments [Member] | Canada [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									4.1	3.5	3.2
Total revenues									4.1	3.5	3.2
Total assets	2.8				3.1				2.8	3.1	2.8
Operating Segments [Member] | New Zealand [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									120.7	118.6	78.4
Total revenues									120.7	118.6	78.4
Total assets	668.9				623.1				668.9	623.1	614.1
Operating Segments [Member] | New Zealand [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									5.1	4.0	2.9
Total revenues									5.1	4.0	2.9
Total assets	4.4				4.1				4.4	4.1	1.6
Operating Segments [Member] | Other Foreign [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									79.7	54.4	43.6
Total revenues									79.7	54.4	43.6
Total assets	331.3				203.0				331.3	203.0	207.2
Operating Segments [Member] | Other Foreign [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									20.9	13.8	11.0
Total revenues									20.9	13.8	11.0
Total assets	$ 17.6				$ 19.1				$ 17.6	$ 19.1	$ 16.9