Arthur J. Gallagher & Co. (AJG) 10-Q 29 Oct 21 2021 Q3 Quarterly report Financial data

Cover Page

Cover Page	9 Months Ended
	Sep. 30, 2021shares
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Period End Date	Sep. 30, 2021
Document Fiscal Period Focus	Q3
Entity Interactive Data Current	Yes
Entity Central Index Key	0000354190
Current Fiscal Year End Date	--12-31
Entity Registrant Name	ARTHUR J. GALLAGHER & CO.
Entity File Number	1-09761
Document Transition Report	false
Entity Tax Identification Number	36-2151613
Entity Incorporation, State or Country Code	DE
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Address, Address Line One	2850 Golf Road
Entity Address, City or Town	Rolling Meadows
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60008
City Area Code	630
Local Phone Number	773-3800
Entity Shell Company	false
Trading Symbol	AJG
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock, par value $1.00 per share
Entity Common Stock, Shares Outstanding	207,278,000
Document Quarterly Report	true

Consolidated Statement of Earni

Consolidated Statement of Earnings - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues before reimbursements	$ 2,105.6	$ 1,807.5	$ 6,135	$ 5,188.3
Reimbursements	32.6	41.6	101.7	111.7
Total revenues	2,138.2	1,849.1	6,236.7	5,300
Compensation	1,000.5	873.5	2,913.2	2,585.3
Operating	265.6	203.2	752.1	675.7
Reimbursements	32.6	41.6	101.7	111.7
Cost of revenues from clean coal activities	366.1	319.2	944	665.8
Interest	60.3	48.1	164.6	148.6
Loss on extinguishment of debt	16.2		16.2
Depreciation	38.4	36.5	113.7	107.7
Amortization	92.9	97.4	306.1	321.2
Change in estimated acquisition earnout payables	34.2	20.1	63	(53.2)
Total expenses	1,906.8	1,639.6	5,374.6	4,562.8
Earnings before income taxes	231.4	209.5	862.1	737.2
Provision (benefit) for income taxes	(7.2)	21.7	28	32.2
Net earnings	238.6	187.8	834.1	705
Net earnings attributable to noncontrolling interests	13.5	11.2	36.7	28.4
Net earnings attributable to controlling interests	$ 225.1	$ 176.6	$ 797.4	$ 676.6
Basic net earnings per share	$ 1.09	$ 0.92	$ 3.97	$ 3.55
Diluted net earnings per share	1.06	0.90	3.88	3.48
Dividends declared per common share	$ 0.48	$ 0.45	$ 1.44	$ 1.35
Commissions [Member]
Revenues before reimbursements	$ 1,016.2	$ 889.9	$ 3,118.7	$ 2,734.6
Broker Fees [Member]
Revenues before reimbursements	601.7	495.8	1,696.4	1,462.8
Supplemental Revenue [Member]
Revenues before reimbursements	61	54.7	183	164
Contingent Revenue [Member]
Revenues before reimbursements	43.7	34.5	150.3	117
Investment Income [Member]
Revenues before reimbursements	20.8	19	56	53.6
Net Gains on Divestitures [Member]
Revenues before reimbursements	4.3	3.4	9	4.6
Clean Coal Activities [Member]
Revenues before reimbursements	357.6	310.8	919.2	652.1
Other Net Revenue (Losses) [Member]
Revenues before reimbursements	$ 0.3	$ (0.6)	$ 2.4	$ (0.4)

Consolidated Statement of Compr

Consolidated Statement of Comprehensive Earnings - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net earnings	$ 238.6	$ 187.8	$ 834.1	$ 705
Change in pension liability, net of taxes	0.6	0.4	1.7	1.6
Foreign currency translation, net of taxes	(98.1)	101.9	(9.7)	(28.7)
Change in fair value of derivative investments, net of taxes	4.8	9.5	28.4	(83.7)
Comprehensive earnings	145.9	299.6	854.5	594.2
Comprehensive earnings attributable to noncontrolling interests	13.8	11.7	37.3	29.3
Comprehensive earnings attributable to controlling interests	$ 132.1	$ 287.9	$ 817.2	$ 564.9

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Cash and cash equivalents	$ 2,735.1	$ 664.6
Restricted cash	3,277.2	2,909.7
Premiums and fees receivable	7,756.5	6,436
Other current assets	1,369.5	1,113.9
Total current assets	15,138.3	11,124.2
Fixed assets - net	454.5	450.7
Deferred income taxes	1,171.4	1,085.8
Other noncurrent assets	854.8	769.9
Right-of-use assets	330.3	373.9
Goodwill	6,627.5	6,127
Amortizable intangible assets - net	2,339	2,399.9
Total assets	26,915.8	22,331.4
Premiums payable to underwriting enterprises	9,161.7	7,784.6
Accrued compensation and other current liabilities	1,536.9	1,596.2
Deferred revenue - current	527.1	475.6
Premium financing debt	234.1	203.6
Corporate related borrowings - current	200	75
Total current liabilities	11,659.8	10,135
Corporate related borrowings - noncurrent	5,072.4	4,266
Deferred revenue - noncurrent	61.1	65.7
Lease liabilities - noncurrent	281.4	320.9
Other noncurrent liabilities	1,371	1,311.1
Total liabilities	18,445.7	16,098.7
Stockholders' equity:
Common stock - issued and outstanding 207.3 shares in 2021 and 193.7 shares in 2020	207.3	193.7
Capital in excess of par value	5,962.3	4,264.4
Retained earnings	2,873.8	2,371.7
Accumulated other comprehensive loss	(623.2)	(643.6)
Stockholders' equity attributable to controlling interests	8,420.2	6,186.2
Stockholders' equity attributable to noncontrolling interests	49.9	46.5
Total stockholders' equity	8,470.1	6,232.7
Total liabilities and stockholders' equity	$ 26,915.8	$ 22,331.4

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - shares shares in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Common stock - issued shares	207.3	193.7
Common stock - outstanding shares	207.3	193.7

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net earnings	$ 834.1	$ 705
Adjustments to reconcile net earnings to net cash provided by operating activities:
Net gain on investments and other	(10.2)	(4.1)
Loss on extinguishment of debt	9.7
Depreciation and amortization	419.8	428.9
Change in estimated acquisition earnout payables	63	(53.2)
Amortization of deferred compensation and restricted stock	48.9	45.1
Stock-based and other noncash compensation expense	9.1	10.1
Payments on acquisition earnouts in excess of original estimates	(29.3)	(14.5)
Effect of changes in foreign exchange rates	3.7	(3)
Net change in premiums and fees receivable	(1,129.7)	(1,297.9)
Net change in deferred revenue	36.2	24.9
Net change in premiums payable to underwriting enterprises	1,176.7	1,596
Net change in other current assets	(142.7)	(24.5)
Net change in accrued compensation and other current liabilities	56.3	(6.4)
Net change in income taxes payable	(106.8)	61.8
Net change in deferred income taxes	(107.3)	(129.9)
Net change in other noncurrent assets and liabilities	9.7	49.3
Net cash provided by operating activities	1,141.2	1,387.6
Cash flows from investing activities:
Capital expenditures	(103.2)	(79.5)
Cash paid for acquisitions, net of cash and restricted cash acquired	(631.6)	(91.3)
Net proceeds from sales of operations/books of business	14.3	9
Net funding of investment transactions	(1.2)	(1)
Net cash used by investing activities	(721.7)	(162.8)
Cash flows from financing activities:
Payments on acquisition earnouts	(130.8)	(32.6)
Proceeds from issuance of common stock	1,524.6	83.6
Payments to noncontrolling interests	(28.7)	(76.4)
Dividends paid	(292.2)	(260.1)
Net borrowings on premium financing debt facility	40.3	16.9
Borrowings on line of credit facility	925	2,630
Repayments on line of credit facility	(925)	(3,150)
Borrowings of corporate related long-term debt	1,667.6	575
Repayments of corporate related long-term debt	(733)	(100.4)
Debt acquisition costs	(14.3)	(1.3)
Settlements on terminated interest rate swaps	(31.9)	(65.9)
Net cash provided (used) by financing activities	2,001.6	(381.2)
Effect of changes in foreign exchange rates on cash and cash equivalents and restricted cash	16.9	7.3
Net increase in cash, cash equivalents and restricted cash	2,438	850.9
Cash, cash equivalents and restricted cash at beginning of period	3,574.3	2,623.9
Cash, cash equivalents and restricted cash at end of period	$ 6,012.3	$ 3,474.8

Consolidated Statement of Stock

Consolidated Statement of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Capital in Excess of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]
Beginning balance at Dec. 31, 2019	$ 5,215.5	$ 188.1	$ 3,825.7	$ 1,901.3	$ (759.6)	$ 60
Beginning balance (in shares) at Dec. 31, 2019		188.1
Net earnings	355.4			346.3		9.1
Net purchase of subsidiary shares from noncontrolling interests	(10.8)					(10.8)
Dividends paid to noncontrolling interests	(18.9)					(18.9)
Net change in pension asset/liability, net of taxes	1.2				1.2
Foreign currency translation	(380.1)				(381.3)	1.2
Change in fair value of derivative instruments, net of taxes	(91.4)				(91.4)
Compensation expense related to stock option plan grants	3.4		3.4
Common stock issued in: purchase transactions	73.1	$ 0.7	72.4
Common stock issued in: purchase transactions (in shares)		0.7
Stock option plans	17.9	$ 0.4	17.5
Stock option plans (in shares)		0.4
Employee stock purchase plan	6	$ 0.1	5.9
Employee stock purchase plan (in shares)		0.1
Deferred compensation and restricted stock	(15)	$ 0.3	(15.3)
Deferred compensation and restricted (in shares)		0.3
Cash dividends declared on common stock	(86.2)			(86.2)
Ending balance at Mar. 31, 2020	5,070.1	$ 189.6	3,909.6	2,161.4	(1,231.1)	40.6
Ending balance, (in shares) at Mar. 31, 2020		189.6
Beginning balance at Dec. 31, 2019	5,215.5	$ 188.1	3,825.7	1,901.3	(759.6)	60
Beginning balance (in shares) at Dec. 31, 2019		188.1
Net earnings	705
Net change in pension asset/liability, net of taxes	1.6
Change in fair value of derivative instruments, net of taxes	(83.7)
Compensation expense related to stock option plan grants	10.1
Ending balance at Sep. 30, 2020	5,818.2	$ 192.3	4,132.5	2,317.3	(870.4)	46.5
Ending balance, (in shares) at Sep. 30, 2020		192.3
Beginning balance at Mar. 31, 2020	5,070.1	$ 189.6	3,909.6	2,161.4	(1,231.1)	40.6
Beginning balance (in shares) at Mar. 31, 2020		189.6
Net earnings	161.8			153.7		8.1
Net purchase of subsidiary shares from noncontrolling interests	4.3					4.3
Dividends paid to noncontrolling interests	(6.3)					(6.3)
Foreign currency translation	249.9				250.7	(0.8)
Change in fair value of derivative instruments, net of taxes	(1.8)				(1.8)
Compensation expense related to stock option plan grants	3.3		3.3
Common stock issued in: purchase transactions	110.9	$ 1.2	109.7
Common stock issued in: purchase transactions (in shares)		1.2
Stock option plans	21.9	$ 0.5	21.4
Stock option plans (in shares)		0.5
Employee stock purchase plan	12.4	$ 0.1	12.3
Employee stock purchase plan (in shares)		0.1
Deferred compensation and restricted stock	(5.2)	$ 0.1	(5.3)
Deferred compensation and restricted (in shares)		0.1
Cash dividends declared on common stock	(86.8)			(86.8)
Ending balance at Jun. 30, 2020	5,534.5	$ 191.5	4,051	2,228.3	(982.2)	45.9
Ending balance, (in shares) at Jun. 30, 2020		191.5
Net earnings	187.8			176.6		11.2
Net purchase of subsidiary shares from noncontrolling interests	(0.8)					(0.8)
Dividends paid to noncontrolling interests	(10.3)					(10.3)
Net change in pension asset/liability, net of taxes	0.4				0.4
Foreign currency translation	102.4				101.9	0.5
Change in fair value of derivative instruments, net of taxes	9.5				9.5
Compensation expense related to stock option plan grants	3.4		3.4
Common stock issued in: purchase transactions	39	$ 0.4	38.6
Common stock issued in: purchase transactions (in shares)		0.4
Stock option plans	17.2	$ 0.3	16.9
Stock option plans (in shares)		0.3
Employee stock purchase plan	8.1	$ 0.1	8
Employee stock purchase plan (in shares)		0.1
Deferred compensation and restricted stock	14.6		14.6
Cash dividends declared on common stock	(87.6)			(87.6)
Ending balance at Sep. 30, 2020	5,818.2	$ 192.3	4,132.5	2,317.3	(870.4)	46.5
Ending balance, (in shares) at Sep. 30, 2020		192.3
Beginning balance at Dec. 31, 2020	$ 6,232.7	$ 193.7	4,264.4	2,371.7	(643.6)	46.5
Beginning balance (in shares) at Dec. 31, 2020	193.7	193.7
Net earnings	$ 393.7			382.1		11.6
Dividends paid to noncontrolling interests	(10.3)					(10.3)
Net change in pension asset/liability, net of taxes	1				1
Foreign currency translation	58.6				58	0.6
Change in fair value of derivative instruments, net of taxes	56.8				56.8
Compensation expense related to stock option plan grants	3.7		3.7
Common stock issued in: purchase transactions	67.5	$ 0.6	66.9
Common stock issued in: purchase transactions (in shares)		0.6
Stock option plans	30.2	$ 0.6	29.6
Stock option plans (in shares)		0.6
Employee stock purchase plan	7	$ 0.1	6.9
Employee stock purchase plan (in shares)		0.1
Shares issued to benefit plans	71.4	$ 0.6	70.8
Shares issued to benefit plans (in shares)		0.6
Deferred compensation and restricted stock	(37.9)	$ 0.3	(38.2)
Deferred compensation and restricted (in shares)		0.3
Cash dividends declared on common stock	(94.9)			(94.9)
Ending balance at Mar. 31, 2021	6,779.5	$ 195.9	4,404.1	2,658.9	(527.8)	48.4
Ending balance, (in shares) at Mar. 31, 2021		195.9
Beginning balance at Dec. 31, 2020	$ 6,232.7	$ 193.7	4,264.4	2,371.7	(643.6)	46.5
Beginning balance (in shares) at Dec. 31, 2020	193.7	193.7
Net earnings	$ 834.1
Net change in pension asset/liability, net of taxes	1.7
Change in fair value of derivative instruments, net of taxes	28.4
Compensation expense related to stock option plan grants	$ 12.9
Stock option plans (in shares)	1.1
Ending balance at Sep. 30, 2021	$ 8,470.1	$ 207.3	5,962.3	2,873.8	(623.2)	49.9
Ending balance, (in shares) at Sep. 30, 2021	207.3	207.3
Beginning balance at Mar. 31, 2021	$ 6,779.5	$ 195.9	4,404.1	2,658.9	(527.8)	48.4
Beginning balance (in shares) at Mar. 31, 2021		195.9
Net earnings	201.8			190.2		11.6
Net purchase of subsidiary shares from noncontrolling interests	(11)					(11)
Dividends paid to noncontrolling interests	(7.4)					(7.4)
Net change in pension asset/liability, net of taxes	0.1				0.1
Foreign currency translation	30.1				30.4	(0.3)
Change in fair value of derivative instruments, net of taxes	(33.2)				(33.2)
Compensation expense related to stock option plan grants	4.5		4.5
Common stock issued in: purchase transactions	13	$ 0.1	12.9
Common stock issued in: purchase transactions (in shares)		0.1
Stock option plans	15.7	$ 0.3	15.4
Stock option plans (in shares)		0.3
Employee stock purchase plan	10.5	$ 0.1	10.4
Employee stock purchase plan (in shares)		0.1
Deferred compensation and restricted stock	12.2	$ 0.1	12.1
Deferred compensation and restricted (in shares)		0.1
Stock issuance from public offering	1,437.9	$ 10.3	1,427.6
Stock issuance from public offering (in shares)		10.3
Other compensation expense	0.3		0.3
Cash dividends declared on common stock	(100)			(100)
Ending balance at Jun. 30, 2021	8,354	$ 206.8	5,887.3	2,749.1	(530.5)	41.3
Ending balance, (in shares) at Jun. 30, 2021		206.8
Net earnings	238.6			225.1		13.5
Net purchase of subsidiary shares from noncontrolling interests	5					5
Dividends paid to noncontrolling interests	(10.2)					(10.2)
Net change in pension asset/liability, net of taxes	0.6				0.6
Foreign currency translation	(97.8)				(98.1)	0.3
Change in fair value of derivative instruments, net of taxes	4.8				4.8
Compensation expense related to stock option plan grants	4.7		4.7
Common stock issued in: purchase transactions	35.1	$ 0.2	34.9
Common stock issued in: purchase transactions (in shares)		0.2
Stock option plans	7.7	$ 0.2	7.5
Stock option plans (in shares)		0.2
Employee stock purchase plan	14.8	$ 0.1	14.7
Employee stock purchase plan (in shares)		0.1
Deferred compensation and restricted stock	13.2		13.2
Cash dividends declared on common stock	(100.4)			(100.4)
Ending balance at Sep. 30, 2021	$ 8,470.1	$ 207.3	$ 5,962.3	$ 2,873.8	$ (623.2)	$ 49.9
Ending balance, (in shares) at Sep. 30, 2021	207.3	207.3

Consolidated Statement of Sto_2

Consolidated Statement of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Statement Of Stockholders Equity [Abstract]
Tax effect on net change in pension asset/liability	$ 0.1	$ 0.1	$ 0.2	$ 0.1		$ 0.3
Net change in fair value of derivative instruments, tax	$ 1.8	$ (10.9)	$ 19	$ 1.6	$ 0	$ (28.9)

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	1. Summary of Significant Accounting Policies Terms Used in Notes to Consolidated Financial Statements ASC - Accounting Standards Codification. ASU - Accounting Standards Update. FASB - The Financial Accounting Standards Board. GAAP - U.S. generally accepted accounting principles. IRC - Internal Revenue Code. IRS - Internal Revenue Service. Underwriting enterprises - Insurance companies, reinsurance companies and various other forms of risk-taking entities, including intermediaries of underwriting enterprises. Nature of Operations and Basis of Presentation Arthur J. Gallagher & Co. and its subsidiaries, collectively referred to herein as we, our, us or the company, provide insurance brokerage, consulting and third party claims settlement and administration services to both domestic and international entities. We have three reportable segments: brokerage, risk management and corporate. Our brokers, agents and administrators act as intermediaries between underwriting enterprises and our clients. Our brokerage segment operations provide brokerage and consulting services to companies and entities of all types, including commercial, not-for-profit, public entities, and, to a lesser extent, individuals, in the areas of insurance placement, risk of loss management, and management of employer sponsored benefit programs. Our risk management segment operations provide contract claim settlement, claim administration, loss control services and risk management consulting for commercial, not-for-profit, captive and public entities, and various other organizations that choose to self-insure property/casualty coverages or choose to use a third‑party claims management organization rather than the claim services provided by underwriting enterprises. The corporate segment reports the financial information related to our debt and other corporate costs, clean energy investments, external acquisition‑related expenses and the impact of foreign currency translation. Clean energy investments consist of our investments in limited liability companies that own 35 commercial clean coal production facilities that can produce refined coal using Chem-Mod LLC’s proprietary technologies. We believe these operations produce refined coal that qualifies for tax credits under IRC Section 45. We do not assume underwriting risk on a net basis, other than with respect to de minimis amounts necessary to provide minimum or regulatory capital to organize captives, pools, specialized underwriters or risk-retention groups. Rather, capital necessary for covering losses is provided by underwriting enterprises. Investment income and other revenues are primarily generated from our premium financing operations, our invested cash and restricted cash we hold on behalf of our clients, as well as clean energy investments. In addition, our share of the net earnings related to partially owned entities that are accounted for using the equity method is included in investment income. We are headquartered in Rolling Meadows, Illinois, have operations in 57 countries and offer client-service capabilities in more than 150 countries globally through a network of correspondent insurance brokers and consultants. We have prepared the accompanying unaudited consolidated financial statements pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and footnote disclosures normally included in annual financial statements have been omitted pursuant to such rules and regulations. The unaudited consolidated financial statements included herein are, in the opinion of management, prepared on a basis consistent with our audited consolidated financial statements for the year ended December 31, 2020, and include all normal recurring adjustments necessary for a fair presentation of the information set forth. The quarterly results of operations are not necessarily indicative of the results of operations to be reported for subsequent quarters or the full year. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2020. In the preparation of our unaudited consolidated financial statements as of September 30, 2021, management evaluated all material subsequent events or transactions that occurred after the balance sheet date through the date on which the financial statements were issued, for potential recognition and/or disclosure therein. Use of Estimates The preparation of our consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. These accounting principles require us to make estimates and assumptions that affect the reported amounts of assets and liabilities and revenues and expenses, and the disclosure of contingent assets and liabilities at the date of our consolidated financial statements. We periodically evaluate our estimates and assumptions, including those relating to the valuation of goodwill and other intangible assets, right-of-use assets, investments (including our IRC Section 45 investments), income taxes, revenue recognition, deferred costs, stock-based compensation, claims handling obligations, retirement plans, litigation and contingencies. We base our estimates on historical experience and various assumptions that we believe to be reasonable based on specific circumstances. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed in the notes herein.

Effect of New Accounting Pronou

Effect of New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2021
Accounting Changes And Error Corrections [Abstract]
Effect of New Accounting Pronouncements	2. Effect of New Accounting Pronouncements Simplifying the Accounting for Income Taxes In December 2019, the FASB issued ASU 2019-12, “Income Taxes (Topic 740): , We adopted this new guidance effective January 1, 2021. The adoption did not have any impact on our consolidated financial statements.

Business Combinations

Business Combinations	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Business Combinations	3. Business Combinations During the nine-month period ended September 30, 2021, we acquired substantially all of the net assets of the following firms in exchange for our common stock and/or cash. These acquisitions have been accounted for using the acquisition method for recording business combinations (in millions, except share data): Total Maximum Common Common Recorded Recorded Potential Name and Effective Shares Share Accrued Escrow Earnout Purchase Earnout Date of Acquisition Issued Value Cash Paid Liability Deposited Payable Price Payable (000s) Atlas General Holdings, LLC January 1, 2021 (AGH) — $ — $ 95.2 $ — $ 5.4 $ 8.7 $ 109.3 $ 35.0 Bollington Wilson Group February 5, 2021 (BWG) — — 328.0 — 1.4 — 329.4 — LDJ American Online Benefits Group, LLC May 1, 2021 (LDJ) — — 38.2 — 7.1 14.8 60.1 20.0 Sixteen other acquisitions completed in 2021 337.0 44.4 157.3 13.1 18.9 34.1 267.8 70.2 337.0 $ 44.4 $ 618.7 $ 13.1 $ 32.8 $ 57.6 $ 766.6 $ 125.2 On August 13, 2021, we announced an agreement to acquire the Willis Towers Watson plc treaty reinsurance brokerage operations for an initial gross consideration of $3.25 billion, and potential additional consideration of $750 million subject to certain third-year revenue targets. We intend to finance the transaction using cash on hand, including the $1.4 billion of net cash raised via the May 17, 2021 follow‑on common stock offering and the $850 million of net cash borrowed via the May 20, 2021 30-year senior note issuance, short-term borrowings and/or capital market transactions. On May 20, 2021, we closed and funded an offering of $1,500.0 million of unsecured senior notes in two tranches. The $650.0 million aggregate principal amount of 2.50% Senior Notes were due 2031 (which we refer to as the 2031 Notes) and the $850.0 million aggregate principal amount of 3.50% Senior Notes are due 2051 (which we refer to as the 2051 Notes). The weighted average interest rate was 3.31% per annum after giving effect to underwriting costs and the net hedge loss. Prior to the termination of our agreement to acquire certain Willis Towers Watson plc brokerage operations, our intent was to use the net proceeds of this offering to fund this transaction. In conjunction with the termination of this agreement, on July 29, 2021, we exercised the special option redemption feature for the 2031 Senior Notes. These notes were redeemed on August 13, 2021 which resulted in a loss on extinguishment of debt of $16.2 million (which is described in further detail in Note 7 below). We now expect to use the net proceeds of this offering related to the 2051 Notes to fund a portion of the cash consideration payable in connection with the Willis Tower Watson plc treaty reinsurance transaction. On May 17, 2021, we closed on a follow-on public offering of our common stock whereby 10.3 million shares of our stock were issued for net proceeds, after underwriting discounts and other expenses related to this offering, of $1,437.9 million. Prior to the termination of our agreement to acquire certain Willis Towers Watson plc brokerage operations, our intent was to use the net proceeds of the offering to fund this acquisition. We now intend to use the net proceeds of the offering to fund the acquisition of the Willis Towers Watson plc treaty reinsurance brokerage operations. Common shares issued in connection with acquisitions are valued at closing market prices as of the effective date of the applicable acquisition or on the days when the shares are issued, if purchase consideration is deferred. We record escrow deposits that are returned to us as a result of adjustments to net assets acquired as reductions of goodwill when the escrows are settled. The maximum potential earnout payables disclosed in the foregoing table represent the maximum amount of additional consideration that could be paid pursuant to the terms of the purchase agreement for the applicable acquisition. The amounts recorded as earnout payables, which are primarily based upon the estimated future operating results of the acquired entities over a two- to three-year period subsequent to the acquisition date, are measured at fair value as of the acquisition date and are included on that basis in the recorded purchase price consideration in the foregoing table. We will record subsequent changes in these estimated earnout obligations, including the accretion of discount, in our consolidated statement of earnings when incurred. The fair value of these earnout obligations is based on the present value of the expected future payments to be made to the sellers of the acquired entities in accordance with the provisions outlined in the respective purchase agreements, which is a Level 3 fair value measurement. In determining fair value, we estimated the acquired entity’s future performance using financial projections developed by management for the acquired entity and market participant assumptions that were derived for revenue growth and/or profitability. Revenue growth rates generally ranged from 2.5% to 15.0% for our 2021 acquisitions. We estimated future payments using the earnout formula and performance targets specified in each purchase agreement and the financial projections just described. We then discounted these payments to present value using a risk-adjusted rate that takes into consideration market-based rates of return that reflect the ability of the acquired entity to achieve the targets. The discount rate was 7.0% to 10.5% for all of our 2021 acquisitions. Changes in financial projections, market participant assumptions for revenue growth and/or profitability, or the risk-adjusted discount rate, would result in a change in the fair value of recorded earnout obligations. During the three-month periods ended September 30, 2021 and 2020 , we recognized $ 8.2 million and $ 5.9 million, respectively, of expense in our consolidated statement of earnings related to the accretion of the discount recorded for earnout obligations in connection with our acquisitions. During the nine-month periods ended September 30, 2021 and 2020 , we recognized $ 26.4 million and $ 24.3 million, respectively, of expense in our consolidated statement of earnings related to the accretion of the discount recorded for earnout obligations in connection with our acquisitions. In addition, during the three-month periods ended September 30, 2021 and 2020 , we recognized $ 26.0 million and $ 14.2 million of expense , respectively, related to net adjustment s in the estimated fair value of the liability for earnout obligation s in connection with revised projections of future performance for 44 and 55 acquisition s , respectively . In addition, during the nine-month periods ended September 30, 2021 and 2020 , we recognized $ 36.6 million of expense and $ 77.4 million of income, respectively, related to net adjustments in the estimated fair value of the liability for earnout obligations in connection with revised projections of future performance for 80 and 126 acquisitions, respectively. The aggregate amount of maximum earnout obligations related to acquisitions was $ 1,026.1 million as of September 30, 2021 , of which $ 578.7 million was recorded in the consolidated balance sheet as of September 30, 2021 , based on the estimated fair value of the expected future payments to be made , of which approximately $ 560.5 million can be settled in cash or stock at our option and $ 18.2 million must be settled in cash . The following is a summary of the estimated fair values of the net assets acquired at the date of each acquisition made in the nine‑month period ended September 30, 2021 (in millions): Sixteen Other AGH BWG LDJ Acquisitions Total Cash and restricted cash $ 32.8 $ 24.9 $ 0.7 $ 11.0 $ 69.4 Other current assets 112.0 15.3 15.8 51.5 194.6 Fixed assets 7.4 3.8 — 0.2 11.4 Noncurrent assets 1.2 5.3 0.2 6.2 12.9 Goodwill 45.9 257.1 25.7 124.8 453.5 Expiration lists 39.7 81.6 25.4 133.2 279.9 Non-compete agreements 0.3 1.2 0.2 2.1 3.8 Trade names — 3.4 0.2 1.1 4.7 Total assets acquired 239.3 392.6 68.2 330.1 1,030.2 Current liabilities 129.8 39.6 8.0 49.9 227.3 Noncurrent liabilities 0.2 23.6 0.1 12.4 36.3 Total liabilities assumed 130.0 63.2 8.1 62.3 263.6 Total net assets acquired $ 109.3 $ 329.4 $ 60.1 $ 267.8 $ 766.6 Among other things, these acquisitions allow us to expand into desirable geographic locations, further extend our presence in the retail and wholesale insurance and reinsurance brokerage markets and in third party claims administration services and increase the volume of general services currently provided. The excess of the purchase price over the estimated fair value of the tangible net assets acquired at the acquisition date was allocated to goodwill, expiration lists, non-compete agreements and trade names in the amounts of $453.5 million, $279.9 million, $3.8 million and $4.7 million, respectively, within the brokerage and risk management segments. Provisional estimates of fair value are established at the time of each acquisition and are subsequently reviewed within the first year of operations subsequent to the acquisition date to determine the necessity for adjustments. The fair value of the tangible assets and liabilities for each applicable acquisition at the acquisition date approximated their carrying values. The fair value of expiration lists was established using the excess earnings method, which is an income approach based on estimated financial projections developed by management for each acquired entity using market participant assumptions. Revenue growth and attrition rates generally ranged from 3.0% to 4.8% and 4.3% to 16.1%, respectively, for our 2020 and 2021 acquisitions for which valuations were performed in 2021. We estimate the fair value as the present value of the benefits anticipated from ownership of the subject expiration list in excess of returns required on the investment in contributory assets necessary to realize those benefits. The rate used to discount the net benefits was based on a risk-adjusted rate that takes into consideration market-based rates of return and reflects the risk of the asset relative to the acquired business. These discount rates generally ranged from 9.0% to 17.0% for our 2020 and 2021 acquisitions for which valuations were performed in 2021. The fair value of non-compete agreements was established using the profit differential method, which is an income approach based on estimated financial projections developed by management for the acquired company using market participant assumptions and various non-compete scenarios. Expiration lists, non-compete agreements and trade names related to our acquisitions are amortized using the straight-line method over their estimated useful lives (two to fifteen years for expiration lists, two to six years for non-compete agreements and two to fifteen years for trade names), while goodwill is not subject to amortization. We use the straight-line method to amortize these intangible assets because the pattern of their economic benefits cannot be reasonably determined with any certainty. We review all of our identifiable intangible assets for impairment periodically (at least annually) and whenever events or changes in business circumstances indicate that the carrying value of the assets may not be recoverable. In reviewing identifiable intangible assets, if the undiscounted future cash flows were less than the carrying amount of the respective (or underlying) asset, an indicator of impairment would exist and further analysis would be required to determine whether or not a loss would need to be charged against current period earnings Of the $ 279.9 million of expiration lists , $ 3.8 million of non-compete agreements and $ 4.7 million of trade names related to our a cquisitions made during the nine-month period ended September 30, 2021 , $ 118.0 million , $ 1.6 million and $ 3.5 million , respectively, is not expected to be deductible for income tax purposes. Accordingly, we recorded a deferred tax liability of $ 20.2 million, and a corresponding amount of goodwill , in the nine-month period ended September 30, 2021 , related to the nondeductible amortizable intangible assets . Our consolidated financial statements for the nine-month period ended September 30, 2021 include the operations of the entities acquired in the nine-month period ended September 30, 2021 from their respective acquisition dates. The following is a summary of the unaudited pro forma historical results, as if these entities had been acquired at January 1, 2020 (in millions, except per share data): Three-month period ended Nine-month period ended September 30, September 30, 2021 2020 2021 2020 Total revenues $ 2,140.7 $ 1,899.6 $ 6,278.4 $ 5,435.7 Net earnings attributable to controlling interests 225.0 181.6 801.9 685.9 Basic net earnings per share 1.09 0.94 3.99 3.60 Diluted net earnings per share 1.06 0.92 3.90 3.52 The unaudited pro forma results above have been prepared for comparative purposes only and do not purport to be indicative of the results of operations which actually would have resulted had these acquisitions occurred at January 1, 2020, nor are they necessarily indicative of future operating results. Annualized revenues of entities acquired during the nine-month period ended September 30, 2021 totaled approximately $189.9 million. For the nine-month period ended September 30, 2021, total revenues and net loss recorded in our unaudited consolidated statement of earnings related to our acquisitions made during the nine-month period ended September 30, 2021, in the aggregate, were $95.8 million and $3.3 million, respectively.

Contracts with Customers

Contracts with Customers	9 Months Ended
	Sep. 30, 2021
Revenue From Contract With Customer [Abstract]
Contracts with Customers	4. Contracts with Customers Contract Assets and Liabilities/Contract Balances Information about unbilled receivables, contract assets and contract liabilities from contracts with customers is as follows (in millions): September 30 2021 December 31, 2020 Unbilled receivables $ 810.6 $ 603.1 Deferred contract costs 76.3 102.0 Deferred revenue 588.2 541.3 The unbilled receivables, which are included in premiums and fees receivable in our consolidated balance sheet, primarily relate to our rights to consideration for work completed but not billed at the reporting date. These are transferred to the receivables when the client is billed. The deferred contract costs represent the costs we incur to fulfill a new or renewal contract with our clients prior to the effective date of the contract. These costs are expensed on the contract effective date. The deferred revenue in the consolidated balance sheet includes amounts that represent the remaining performance obligations under our contracts and amounts collected related to advanced billings and deposits received from customers that may or may not ultimately be recognized as revenues in the future. Deposits received from customers could be returned to the customers based on lesser actual transactional volume than originally billed volume. Significant changes in the deferred revenue balances, which include foreign currency translation adjustments, during the period are as follows (in millions): Risk Brokerage Management Total Deferred revenue at December 31, 2020 $ 354.7 $ 186.6 $ 541.3 Incremental deferred revenue 283.3 77.4 360.7 Revenue recognized during the nine-month period ended September 30, 2021 included in deferred revenue at December 31, 2020 (256.5 ) (76.8 ) (333.3 ) Net change in collected billings/deposits received from customers 1.5 7.3 8.8 Impact of change in foreign exchange rates 1.0 — 1.0 Deferred revenue recognized from business acquisitions 9.7 — 9.7 Deferred revenue at September 30, 2021 $ 393.7 $ 194.5 $ 588.2 Revenue recognized during the nine-month period ended September 30, 2021 in the table above included revenue from 2020 acquisitions that would not be reflected in prior periods. Remaining Performance Obligations Remaining performance obligations represent the portion of the contract price for which work has not been performed. As of September 30, 2021, the aggregate amount of the contract price allocated to remaining performance obligations was $588.2 million. The estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period is as follows (in millions): Brokerage Risk Management Total 2021 (remaining three months) $ 235.6 $ 89.4 $ 325.0 2022 135.8 54.6 190.4 2023 19.3 23.4 42.7 2024 1.5 10.6 12.1 2025 0.8 5.9 6.7 Thereafter 0.7 10.6 11.3 Total $ 393.7 $ 194.5 $ 588.2 Deferred Contract Costs We capitalize costs incurred to fulfill contracts as deferred contract costs which are included in other current assets in our consolidated balance sheet. Deferred contract costs were $76.3 million and $102.0 million as of September 30, 2021 and December 31, 2020, respectively. Capitalized fulfillment costs are amortized to expense on the contract effective date. The amount of amortization of the deferred contract costs was $328.5 million and $304.0 million for the nine-month periods ended September 30, 2021 and 2020, respectively. We have applied the practical expedient to recognize the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that we otherwise would have recognized is one year or less for our brokerage segment. These costs are included in compensation and operating expenses in our consolidated statement of earnings.

Other Financial Data

Other Financial Data	9 Months Ended
	Sep. 30, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Other Financial Data	5. Other Financial Data Other Current Assets Major classes of other current assets consist of the following (in millions): September 30, 2021 December 31, 2020 Premium finance advances and loans $ 511.8 $ 442.7 Accrued supplemental, direct bill and other receivables 546.2 361.7 Refined coal production related receivables 108.5 95.4 Deferred contract costs 76.3 102.0 Prepaid expenses 126.7 112.1 Total other current assets $ 1,369.5 $ 1,113.9 The premium finance advances and loans represent short-term loans which we make to many of our brokerage related clients and other non‑brokerage clients to finance their premiums paid to underwriting enterprises. These premium finance advances and loans are primarily generated by three Australian and New Zealand premium finance subsidiaries

Intangible Assets

Intangible Assets	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible Assets	6. Intangible Assets The carrying amount of goodwill at September 30, 2021 and December 31, 2020 allocated by domestic and foreign operations is as follows (in millions): Brokerage Risk Management Corporate Total At September 30, 2021 United States $ 3,568.9 $ 64.8 $ — $ 3,633.7 United Kingdom 1,606.4 15.4 — 1,621.8 Canada 523.0 — — 523.0 Australia 450.0 11.0 — 461.0 New Zealand 220.5 10.6 — 231.1 Other foreign 154.0 — 2.9 156.9 Total goodwill $ 6,522.8 $ 101.8 $ 2.9 $ 6,627.5 At December 31, 2020 United States $ 3,399.8 $ 33.2 $ — $ 3,433.0 United Kingdom 1,328.3 15.1 — 1,343.4 Canada 492.9 — — 492.9 Australia 462.1 11.5 — 473.6 New Zealand 221.9 10.7 — 232.6 Other foreign 148.6 — 2.9 151.5 Total goodwill $ 6,053.6 $ 70.5 $ 2.9 $ 6,127.0 The changes in the carrying amount of goodwill for the nine-month period ended September 30, 2021 are as follows (in millions): Brokerage Risk Management Corporate Total Balance as of December 31, 2020 $ 6,053.6 $ 70.5 $ 2.9 $ 6,127.0 Goodwill acquired during the period 421.9 31.6 — 453.5 Goodwill adjustments due to appraisals and other acquisition adjustments 50.5 — — 50.5 Goodwill written-off related to sales of business and impairment (1.0 ) — — (1.0 ) Foreign currency translation adjustments during the period (2.2 ) (0.3 ) — (2.5 ) Balance as of September 30, 2021 $ 6,522.8 $ 101.8 $ 2.9 $ 6,627.5 Major classes of amortizable intangible assets at September 30, 2021 and December 31, 2020 consist of the following (in millions): September 30, December 31, 2021 2020 Expiration lists $ 4,997.2 $ 4,753.2 Accumulated amortization - expiration lists (2,727.5 ) (2,436.7 ) 2,269.7 2,316.5 Non-compete agreements 73.5 75.9 Accumulated amortization - non-compete agreements (61.9 ) (57.8 ) 11.6 18.1 Trade names 105.5 107.8 Accumulated amortization - trade names (47.8 ) (42.5 ) 57.7 65.3 Net amortizable assets $ 2,339.0 $ 2,399.9 Estimated aggregate amortization expense for each of the next five years and thereafter is as follows (in millions): 2021 (remaining three months) $ 96.0 2022 367.7 2023 342.4 2024 306.1 2025 266.2 Thereafter 960.6 Total $ 2,339.0

Credit and Other Debt Agreement

Credit and Other Debt Agreements	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Credit and Other Debt Agreements	7. Credit and Other Debt Agreements The following is a summary of our corporate and other debt (in millions): September 30, December 31, 2021 2020 Senior Notes: Semi-annual payments of interest, fixed rate of 3.50%, balloon due May 20, 2051 850.0 — Total Senior Notes 850.0 — Note Purchase Agreements: Semi-annual payments of interest, fixed rate of 5.18%, balloon due February 10, 2021 — 75.0 Semi-annual payments of interest, fixed rate of 3.69%, balloon due June 14, 2022 200.0 200.0 Semi-annual payments of interest, fixed rate of 5.49%, balloon due February 10, 2023 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.13%, balloon due June 24, 2023 200.0 200.0 Semi-annual payments of interest, fixed rate of 4.72%, balloon due February 13, 2024 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.58%, balloon due February 27, 2024 325.0 325.0 Quarterly payments of interest, floating rate of 90 day LIBOR plus 1.40%, balloon due June 13, 2024 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.31%, balloon due June 24, 2025 200.0 200.0 Semi-annual payments of interest, fixed rate of 4.85%, balloon due February 13, 2026 140.0 140.0 Semi-annual payments of interest, fixed rate of 4.73%, balloon due February 27, 2026 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.40%, balloon due June 2, 2026 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.36%, balloon due June 24, 2026 150.0 150.0 Semi-annual payments of interest, fixed rate of 3.75%, balloon due January 30, 2027 30.0 30.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due June 27, 2027 125.0 125.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due August 2, 2027 125.0 125.0 Semi-annual payments of interest, fixed rate of 4.14%, balloon due August 4, 2027 98.0 98.0 Semi-annual payments of interest, fixed rate of 3.46%, balloon due December 1, 2027 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.55%, balloon due June 2, 2028 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due June 13, 2028 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.04%, balloon due February 13, 2029 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.98%, balloon due February 27, 2029 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.19%, balloon due June 27, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.19%, balloon due August 2, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 3.48%, balloon due December 2, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 3.99%, balloon due January 30, 2030 341.0 341.0 Semi-annual payments of interest, fixed rate of 4.44%, balloon due June 13, 2030 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.14%, balloon due March 13, 2031 180.0 180.0 Semi-annual payments of interest, fixed rate of 4.70%, balloon due June 2, 2031 25.0 25.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due January 30, 2032 69.0 69.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due June 27, 2032 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due August 2, 2032 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.59%, balloon due June 13, 2033 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.29%, balloon due March 13, 2034 40.0 40.0 Semi-annual payments of interest, fixed rate of 4.48%, balloon due June 12, 2034 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.24%, balloon due January 30, 2035 79.0 79.0 Semi-annual payments of interest, fixed rate of 2.44%, balloon due February 10, 2036 100.0 — Semi-annual payments of interest, fixed rate of 2.46%, balloon due May 5, 2036 75.0 — Semi-annual payments of interest, fixed rate of 4.69%, balloon due June 13, 2038 75.0 75.0 Semi-annual payments of interest, fixed rate of 5.45%, balloon due March 13, 2039 40.0 40.0 Semi-annual payments of interest, fixed rate of 4.49%, balloon due January 30, 2040 56.0 56.0 Total Note Purchase Agreements 4,448.0 4,348.0 Credit Agreement: Periodic payments of interest and principal, prime or LIBOR plus up to 1.45%, expires June 7, 2024 — — Premium Financing Debt Facility - expires September 15, 2023: Facility B AUD denominated tranche, interbank rates plus 1.200% 211.9 193.1 NZD denominated tranche, interbank rates plus 1.650% — — Facility C and D AUD denominated tranche, interbank rates plus 0.680% 11.8 — NZD denominated tranche, interbank rates plus 0.940% 10.4 10.5 Total Premium Financing Debt Facility 234.1 203.6 Total corporate and other debt 5,532.1 4,551.6 Less unamortized debt acquisition costs on Senior Notes and Note Purchase Agreements (15.6 ) (7.0 ) Less unamortized discount on Bonds Payable (10.0 ) — Net corporate and other debt $ 5,506.5 $ 4,544.6 On May 20, 2021, we closed and funded an offering of $1,500.0 million of unsecured senior notes in two tranches. The $650.0 million aggregate principal amount of 2.50% Senior Notes were due 2031 (which we refer to as the 2031 Notes) and the $850.0 million aggregate principal amount of 3.50% Senior Notes are due 2051 (which we refer to as the 2051 Notes, and together with the 2031 Notes, the “Notes”). The weighted average interest rate was 3.31 % per annum after giving effect to underwriting costs and the net hedge loss. In 2018 and 2019, we entered into a pre-issuance interest rate hedging transaction related to these notes . We realized a net cash loss of approximately $ 57.8 million on the hedging transactions that will be recognized on a pro rata basis as an increase to our reported interest expense over a ten year period. Prior to the termination our agreement to acquire certain Willis Towers Watson plc brokerage operations , our intent was to use the net proceeds of this offering to fund a portion of the cash consideration payable in connection with this transaction. We now expect to use the net proceeds of this offering related to the 2051 Notes to fund a portion of the cash consideration payable in connection with the Willis Tower Watson plc treaty reinsurance transaction. The offering of the Notes was made pursuant to a shelf registration statement filed with the SEC. The 2031 Notes had a special optional redemption whereby we had the option to redeem the 2031 Notes, in whole and not in part, by providing notice of such redemption to the holders of the 2031 Notes within 30 days following a Willis Tower Watson plc transaction termination event, at a redemption price equal to 101% of the aggregate principal amount of the 2031 Notes, plus any accrued and unpaid interest. In conjunction with the termination of the Willis Towers Watson plc brokerage acquisition agreement, on July 29, 2021, we exercised the special option redemption feature for the 2031 Senior Notes. These notes were redeemed on August 13, 2021. As a result of the redemption of this debt, we incurred a loss on extinguishment of debt of $16.2 million, which included the redemption price premium of $6.5 million, the unamortized discount amount on the debt issuance and the write-off of all the debt acquisition costs of $9.7 million. The 2051 Notes are not subject to the special optional redemption.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Earnings Per Share	8. The following table sets forth the computation of basic and diluted net earnings per share (in millions, except per share data): Three-month period ended Nine-month period ended September 30, September 30, 2021 2020 2021 2020 Net earnings attributable to controlling interests $ 225.1 $ 176.6 $ 797.4 $ 676.6 Weighted average number of common shares outstanding 207.0 191.9 201.0 190.4 Dilutive effect of stock options using the treasury stock method 4.5 4.4 4.5 4.1 Weighted average number of common and common equivalent shares outstanding 211.5 196.3 205.5 194.5 Basic net earnings per share $ 1.09 $ 0.92 $ 3.97 $ 3.55 Diluted net earnings per share $ 1.06 $ 0.90 $ 3.88 $ 3.48 Anti-dilutive stock-based awards of 1.6 million shares were outstanding at September 30, 2021 that were excluded in the computation of the dilutive effect of stock-based awards for the three-month period then ended. There were no anti-dilutive stock-based awards outstanding at September 30, 2020 that were excluded in the computation of the dilutive effect of stock-based awards for the three‑month period then ended. Anti-dilutive stock-based awards of 1.2 million and 1.0 million shares were outstanding at September 30, 2021 and 2020 that were excluded in the computation of the dilutive effect of stock-based awards for the nine-month periods then ended. These stock‑based awards were excluded from the computation because the exercise prices on these stock-based awards were greater than the average market price of our common shares during the respective period, and therefore, would be anti‑dilutive to earnings per share under the treasury stock method.

Stock Option Plans

Stock Option Plans	9 Months Ended
	Sep. 30, 2021
Text Block [Abstract]
Stock Option Plans	9. Stock Option Plans On May 16, 2017, our stockholders approved the Arthur J. Gallagher & Co. 2017 Long-Term Incentive Plan (which we refer to as the LTIP), which replaced our previous stockholder-approved Arthur J. Gallagher & Co. 2014 Long-Term Incentive Plan (which we refer to as the 2014 LTIP). The LTIP term began May 16, 2017 and terminates on the date of the annual meeting of stockholders in 2027, unless terminated earlier by our board of directors. All of our officers, employees and non-employee directors are eligible to receive awards under the LTIP. The compensation committee of our board of directors determines the annual number of shares delivered under the LTIP. The LTIP provides for non-qualified and incentive stock options, stock appreciation rights, restricted stock and restricted stock units, any or all of which may be made contingent upon the achievement of performance criteria. Shares of our common stock available for issuance under the LTIP include authorized and unissued shares of common stock or authorized and issued shares of common stock reacquired and held as treasury shares or otherwise, or a combination thereof. The number of available shares will be reduced by the aggregate number of shares that become subject to outstanding awards granted under the LTIP. To the extent that shares subject to an outstanding award granted under either the LTIP or prior equity plans are not issued or delivered by reason of the expiration, termination, cancellation or forfeiture of such award or by reason of the settlement of such award in cash, then such shares will again be available for grant under the LTIP. The maximum number of shares available under the LTIP for restricted stock, restricted stock unit awards and performance unit awards settled with stock (i.e., all awards other than stock options and stock appreciation rights) is 1.8 million at September 30, 2021. The LTIP provides for the grant of stock options, which may be either tax-qualified incentive stock options or non-qualified options and stock appreciation rights. The compensation committee determines the period for the exercise of a non-qualified stock option, tax-qualified incentive stock option or stock appreciation right, provided that no option can be exercised later than seven years after its date of grant. The exercise price of a non-qualified stock option or tax-qualified incentive stock option and the base price of a stock appreciation right cannot be less than 100% of the fair market value of a share of our common stock on the date of grant, provided that the base price of a stock appreciation right granted in tandem with an option will be the exercise price of the related option. Upon exercise, the option exercise price may be paid in cash, by the delivery of previously owned shares of our common stock, through a net-exercise arrangement, or through a broker-assisted cashless exercise arrangement. The compensation committee determines all of the terms relating to the exercise, cancellation or other disposition of an option or stock appreciation right upon a termination of employment, whether by reason of disability, retirement, death or any other reason. Stock option and stock appreciation right awards under the LTIP are non-transferable. On March 16, 2021, the compensation committee granted 1,640,000 options under the LTIP to our officers and key employees that become exercisable at the rate of 34%, 33% and 33% on the anniversary date of the grant in 2024, 2025 and 2026, respectively. On March 12, 2020, the compensation committee granted 1,590,740 options under the LTIP to our officers and key employees that become exercisable at the rate of 34%, 33% and 33% on the anniversary date of the grant in 2023, 2024 and 2025, respectively. The 2021 and 2020 options expire seven years from the date of grant, or earlier in the event of certain terminations of employment. For our executive officers age 55 or older, stock options are not subject to forfeiture upon such officers’ departure from the company after two years from the date of grant. During the three-month periods ended September 30, 2021 and 2020, we recognized $4.7 For purposes of expense recognition, the estimated fair values of the stock option grants are amortized to expense over the options’ vesting period. We estimated the fair value of stock options at the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions: 2021 2020 Expected dividend yield 1.5 % 2.1 % Expected risk-free interest rate 0.9 % 0.7 % Volatility 22.9 % 17.3 % Expected life (in years) 5.4 5.4 Option valuation models require the input of highly subjective assumptions including the expected stock price volatility. The Black-Scholes option pricing model was developed for use in estimating the fair value of traded options which have no vesting restrictions and are fully transferable. The weighted average fair value per option for all options granted during the nine-month periods ended September 30, 2021 and 2020, as determined on the grant date using the Black-Scholes option pricing model, was $23.38 and $9.99, respectively. The following is a summary of our stock option activity and related information for 2021 (in millions, except exercise price and year data): Nine-month period ended September 30, 2021 Weighted Average Weighted Remaining Shares Average Contractual Aggregate Under Exercise Term Intrinsic Option Price (in years) Value Beginning balance 7.5 $ 65.09 Granted 1.6 127.90 Exercised (1.1 ) 49.30 Forfeited or canceled (0.1 ) 86.21 Ending balance 7.9 $ 80.17 4.05 $ 539.1 Exercisable at end of period 2.3 $ 51.05 1.76 $ 228.0 Ending unvested and expected to vest 5.1 $ 91.53 4.97 $ 293.8 Options with respect to 9.5 The total intrinsic value of options exercised during the nine-month periods ended September 30, 2021 and 2020 was $93.6 million and $68.7 million, respectively. As of September 30, 2021, we had approximately $52.4 million of total unrecognized compensation expense related to nonvested options. We expect to recognize that cost over a weighted average period of approximately four years. Other information regarding stock options outstanding and exercisable at September 30, 2021 is summarized as follows (in millions, except exercise price and year data): Options Outstanding Options Exercisable Weighted Average Remaining Weighted Weighted Contractual Average Average Number Term Exercise Number Exercise Range of Exercise Prices Outstanding (in years) Price Exercisable Price $ 43.71 — $ 43.71 1.0 $ 1.46 $ 43.71 0.9 43.71 46.17 — 46.17 0.5 0.44 46.17 0.5 46.17 49.55 — 56.86 1.1 2.46 56.83 0.6 56.81 70.74 70.74 1.0 3.46 70.74 0.3 70.74 79.59 — 79.59 1.2 4.45 79.59 — 79.59 86.17 — 86.17 1.5 5.45 86.17 — 86.17 127.90 — 127.90 1.6 6.46 127.90 — — $ 43.71 — $ 127.90 7.9 4.05 $ 80.17 2.3 $ 51.05

Deferred Compensation

Deferred Compensation	9 Months Ended
	Sep. 30, 2021
Compensation Related Costs [Abstract]
Deferred Compensation	10. Deferred Compensation We have a Deferred Equity Participation Plan (which we refer to as the DEPP), which is a non-qualified plan that generally provides for distributions to certain of our key executives when they reach age 62 (or the one-year anniversary of the date of the grant for participants over the age of 61 as of the grant date) or upon or after their actual retirement if later. Under the provisions of the DEPP, we typically contribute cash in an amount approved by the compensation committee to a rabbi trust on behalf of the executives participating in the DEPP, and instruct the trustee to acquire a specified number of shares of our common stock on the open market or in privately negotiated transactions based on participant elections. Distributions under the DEPP may not normally be made until the participant reaches age 62 (or the one-year anniversary of the date of the grant for participants over the age of 61 as of the grant date) and are subject to forfeiture in the event of voluntary termination of employment prior to then. DEPP awards are generally made annually in the first quarter. In addition, we annually make awards under sub-plans of the DEPP for certain production staff, which generally provide for vesting and/or distributions no sooner than five years from the date of awards, although certain awards vest and/or distribute after the earlier of fifteen years or the participant reaching age 65. All contributions to the plan (including sub-plans) deemed to be invested in shares of our common stock are distributed in the form of our common stock and all other distributions are paid in cash. Our common stock that is issued to or purchased by the rabbi trust as a contribution under the DEPP is valued at historical cost , which equals its fair market value at the date of grant or date of purchase . When common stock is issued, w e record an unearned deferred compensation obligation as a reduction of capital in excess of par value in the accompanying consolidated balance sheet , which is amortized to compensation expense ratably over the vesting period of the participants. Future changes in the fair market value of our common stock owed to the participants do not have any impact on the amounts recorded in our consolidated financial statements. In the first quarters of 2021 and 2020, the compensation committee approved $17.0 million and $14.1 million, respectively, of awards in the aggregate to certain key executives under the DEPP that were contributed to the rabbi trust in the first quarters of 2021 and 2020, respectively. We contributed cash to the rabbi trust and instructed the trustee to acquire a specified number of shares of our common stock on the open market to fund these 2021 and 2020 awards. During the three-month periods ended September 30, 2021 and 2020, we charged $3.8 million and $3.0 million, respectively, to compensation expense related to these awards. During the nine‑month periods ended September 30, 2021 and 2020, we charged $10.3 million and $9.0 million, respectively, to compensation expense related to these awards. In the first quarters of 2021 and 2020, the compensation committee approved $3.2 million and $1.8 million, respectively, of awards under the sub-plans referred to above, which were contributed to the rabbi trust in the first quarters of 2021 and 2020, respectively. During the three-month periods ended September 30, 2021 and 2020, we charged $0.5 million and $0.7 million, respectively, to compensation expense related to these awards. During the nine-month periods ended September 30, 2021 and 2020, we charged $1.9 million and $2.1 million, respectively, to compensation expense related to these awards. There were no distributions from the sub-plans during the nine-month periods ended September 30, 2021 and 2020. At September 30, 2021 and December 31, 2020, we recorded $74.3 million (related to 2.7 million shares) and $60.5 million (related to 2.6 million shares), respectively, of unearned deferred compensation as a reduction of capital in excess of par value in the accompanying consolidated balance sheet. The total intrinsic value of our unvested equity-based awards under the plan at September 30, 2021 and December 31, 2020 was $402.8 million and $327.8 million, respectively. During the nine-month period ended September 30, 2021, cash and equity awards with an aggregate fair value of $10.1 million were vested and distributed to executives under the DEPP. During the nine-month period ended September 30, 2020, cash and equity awards with an aggregate fair value of $9.8 million were vested and distributed to executives under the DEPP. We have a Deferred Cash Participation Plan (which we refer to as the DCPP), which is a non-qualified deferred compensation plan for certain key employees, other than executive officers, that generally provides for vesting and/or distributions no sooner than five years from the date of awards. Under the provisions of the DCPP, we typically contribute cash in an amount approved by the compensation committee to the rabbi trust on behalf of the executives participating in the DCPP, and instruct the trustee to acquire a specified number of shares of our common stock on the open market or in privately negotiated transactions based on participant elections. In the first quarters of 2021 and 2020, the compensation committee approved $7.2 million and $3.0 million, respectively, of awards in the aggregate to certain key executives under the DCPP that were contributed to the rabbi trust in the second quarters of 2021 and 2020, respectively. During the three-month periods ended September 30, 2021 and 2020, we charged $2.9 million and $1.6 million, respectively, to compensation expense related to these awards. During the nine-month periods ended September 30, 2021 and 2020, we charged $7.0 million and $5.0 million, respectively, to compensation expense related to these awards. There were $6.7 million and $7.3 million of distributions from the DCPP during the nine-month periods ended September 30, 2021 and 2020, respectively.

Restricted Stock, Performance S

Restricted Stock, Performance Share and Cash Awards	9 Months Ended
	Sep. 30, 2021
Text Block [Abstract]
Restricted Stock, Performance Share and Cash Awards	11. Restricted Stock, Performance Share and Cash Awards Restricted Stock Awards As discussed in Note 9 to these unaudited consolidated financial statements, on May 16, 2017, our stockholders approved the LTIP, which replaced our previous stockholder-approved 2014 LTIP. The LTIP provides for the grant of a stock award either as restricted stock or as restricted stock units to officers, employees and non-employee directors. In either case, the compensation committee may determine that the award will be subject to the attainment of performance measures over an established performance period. Stock awards and the related dividend equivalents are non-transferable and subject to forfeiture if the holder does not remain continuously employed with us during the applicable restriction period or, in the case of a performance-based award, if applicable performance measures are not attained. The compensation committee will determine all of the terms relating to the satisfaction of performance measures and the termination of a restriction period, or the forfeiture and cancellation of a restricted stock award upon a termination of employment, whether by reason of disability, retirement, death or any other reason. The agreements awarding restricted stock units under the LTIP will specify whether such awards may be settled in shares of our common stock, cash or a combination of shares and cash and whether the holder will be entitled to receive dividend equivalents, on a current or deferred basis, with respect to such award. Prior to the settlement of a restricted stock unit, the holder of a restricted stock unit will have no rights as a stockholder of the company . T he maximum number of shares available under the LTIP for restricted stock, restricted stock unit s and performance unit awards settled with stock (i.e., all awards other than stock options and stock appreciation rights) is 4.0 million . At September 30, 2021 , 1.8 million shares were available for grant under the LTIP for such awards . In the first quarters of 2021 and 2020, we granted 314,000 and 405,900 restricted stock units, respectively, to employees under the LTIP, with an aggregate fair value of $40.1 million and $34.9 million, respectively, at the date of grant. These 2021 and 2020 awards of restricted stock units vest in full based on continued employment through March 16, 2026 and March 12, 2025, respectively. For our executive officers age 55 or older, restricted stock units are not subject to forfeiture upon such officers’ departure from the company after two years from the date of grant. We account for restricted stock awards at historical cost, which equals its fair market value at the date of grant, which is amortized to compensation expense ratably over the vesting period of the participants. Future changes in the fair value of our common stock that is owed to the participants do not have any impact on the amounts recorded in our consolidated financial statements. During the three-month periods ended September 30, 2021 and 2020, we recognized $6.7 million and $5.5 million, respectively, to compensation expense related to restricted stock unit awards granted in 2015 through 2021. During the nine-month periods ended September 30, 2021 and 2020, we recognized $20.2 million and $17.0 million, respectively, to compensation expense related to restricted stock unit awards granted in 2015 through 2021. The total intrinsic value of unvested restricted stock units at September 30, 2021 and 2020 was $293.4 million and $221.1 million, respectively. During the three-month period ended September 30, 2021, equity awards (including accrued dividends) with an aggregate value of $0.5 million were vested and distributed to employees under this plan. During the nine‑month periods ended September 30, 2021 and 2020, equity awards (including accrued dividends) with an aggregate value of $48.6 million and $30.6 million, respectively, were vested and distributed to employees under this plan. Performance Share Awards On March 16, 2021 and March 12, 2020, pursuant to the LTIP, the compensation committee approved 67,000 and 82,500, respectively, of provisional performance share awards, with an aggregate fair value of $8.6 million and $7.1 million, respectively, for future grants to our officers. Each performance share award was equivalent to the value of one share of our common stock on the date such provisional award was approved. At the end of the performance period, eligible participants will receive a number of earned shares based on the growth in adjusted EBITDAC per share (as defined in our 2021 Proxy Statement). Earned shares for the 2021 and 2020 provisional awards will fully vest based on continuous employment through March 16, 2024 and March 12, 2023, respectively, and will be settled in unrestricted shares of our common stock on a one-for-one basis as soon as practicable thereafter. The 2021 and 2020 awards are subject to a three-year performance period that began on January 1, 2021 and 2020, respectively, and vest on the three-year anniversary of the date of grant (March 16, 2024 and March 12, 2023). For certain of our executive officers age 55 or older, awards are no longer subject to forfeiture upon such officers’ departure from the company after two years from the date of grant. During the three-month periods ended September 30, 2021 and 2020, we recognized $3.1 million and $3.1 million, respectively, to compensation expense related to performance share awards granted in 2017 through 2021. During the nine-month periods ended September 30, 2021 and 2020, we recognized $9.3 million and $11.6 million, respectively, to compensation expense related to performance share awards granted in 2017 through 2021. The total intrinsic value of unvested performance share awards at September 30, 2021 and 2020 was $48.3 million and $35.4 million, respectively. During the nine-month periods ended September 30, 2021 and 2020, equity awards (including accrued dividends) with an aggregate fair value of $19.1 million and $12.5 million, respectively, were vested and distributed to employees under this plan. Cash Awards On March 16, 2021, pursuant to our Performance Unit Program (which we refer to as the Program), the compensation committee approved provisional cash awards of $18.8 million in the aggregate for future grants to our officers and key employees that are denominated in units (147,000 units in the aggregate), each of which was equivalent to the value of one share of our common stock on the date the provisional award was approved. The Program consists of a one-year three-year On March 12, 2020, pursuant to the Program, the compensation committee approved provisional cash awards of $18.4 million in the aggregate for future grants to our officers and key employees that are denominated in units (213,000 units in the aggregate), each of which was equivalent to the value of one share of our common stock on the date the provisional award was approved. Terms of the 2020 provisional awards were similar to the terms of the 2021 provisional awards. Based on our performance for 2020, we granted 208,000 units under the Program in the first quarter of 2021 that will fully vest on January 1, 2023. During the three and nine-month periods ended September 30, 2021, we recognized $3.2 million and $9.4 million, respectively, to compensation expense related to these awards. On March 14, 2019, pursuant to the Program, the compensation committee approved provisional cash awards of $16.5 million in the aggregate for future grants to our officers and key employees that are denominated in units (206,800 units in the aggregate), each of which was equivalent to the value of one share of our common stock on the date the provisional award was approved. Terms of the 2019 provisional awards were similar to the terms of the 2020 provisional awards. Based on our performance for 2019, we granted 200,000 units under the Program in the first quarter of 2020 that will fully vest on January 1, 2022. During the three-month periods ended September 30, 2021 and 2020, we recognized $2.8 million and $2.9 million to compensation expense related to these awards, respectively. During the nine-month periods ended September 30, 2021 and 2020, we recognized $8.8 million and $7.2 million, respectively, to compensation expense related to these awards, On March 15, 2018, pursuant to the Program, the compensation committee approved provisional cash awards of $15.0 million in the aggregate for future grants to our officers and key employees denominated in units (219,000 units in the aggregate), each of which was equivalent to the value of one share of our common stock on the date the provisional award was approved. Terms of the 2018 provisional awards were similar to the terms of the 2019 provisional awards. Based on our performance, we granted 190,000 units under the Program in the first quarter of 2019 that fully vested on January 1, 2021. During the three and nine-month periods ended September 30, 2020, we recognized $2.7 million and $6.8 million, respectively, to compensation expense related to these awards. During the nine-month period ended September 30, 2021, cash awards related to the 2018 provisional award with an aggregate fair value of $17.7 million (176,300 units in the aggregate) were vested and distributed to employees under the Program. During the nine-month period ended September 30, 2020, cash awards related to the 2017 provisional award with an aggregate fair value of $18.9 million (221,600 units in the aggregate) were vested and distributed to employees under the Program.

Investments

Investments	9 Months Ended
	Sep. 30, 2021
Equity Method Investments And Joint Ventures [Abstract]
Investments	12. Investments The following is a summary of our investments included in other noncurrent assets in the consolidated balance sheet and the related funding commitments (in millions): September 30, 2021 December 31, 2020 Funding Assets Commitments Assets Chem-Mod LLC $ 4.0 $ — $ 4.0 Chem-Mod International LLC 2.0 — 2.0 Clean-coal investments: Non-controlling interest in a limited liability company that owns one 2011 Era Clean Coal Plant — — 0.1 Controlling interest in limited liability companies that own twenty 2011 Era Clean Coal Plants 2.6 — 12.4 Other investments 5.5 0.5 3.9 Total investments $ 14.1 $ 0.5 $ 22.4

Derivatives and Hedging Activit

Derivatives and Hedging Activity	9 Months Ended
	Sep. 30, 2021
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivatives and Hedging Activity	13. Derivatives and Hedging Activity We are exposed to market risks, including changes in foreign currency exchange rates and interest rates. To manage the risk related to these exposures, we enter into various derivative instruments that reduce these risks by creating offsetting exposures. We generally do not enter into derivative transactions for trading or speculative purposes. Foreign Exchange Risk Management We are exposed to foreign exchange risk when we earn revenues, pay expenses, or enter into monetary intercompany transfers denominated in a currency that differs from our functional currency, or other transactions that are denominated in a currency other than our functional currency. We use foreign exchange derivatives, typically forward contracts and options, to reduce our overall exposure to the effects of currency fluctuations on cash flows. These exposures are hedged, on average, for less than three years. Interest Rate Risk Management We enter into various long-term debt agreements. We use interest rate derivatives, typically swaps, to reduce our exposure to the effects of interest rate fluctuations on the forecasted interest rates for up to three years into the future. We have not received or pledged any collateral related to derivative arrangements at September 30, 2021. The notional and fair values of derivatives designated as hedging instruments are as follows at September 30, 2021 and December 31, 2020 (in millions): Derivative Assets Derivative Liabilities Notional Balance Sheet Fair Balance Sheet Fair Instrument Amount Classification Value Classification Value At September 30, 2021 Interest rate contracts $ 1,000.0 Other current assets $ 5.9 Accrued compensation and other current liabilities $ 0.3 Other noncurrent assets 5.9 Other noncurrent liabilities 4.2 Foreign exchange contracts (1) 22.5 Other current assets 4.5 Accrued compensation and other current liabilities 0.9 Other noncurrent assets 1.9 Other noncurrent liabilities 4.4 Total $ 1,022.5 $ 18.2 $ 9.8 At December 31, 2020 Interest rate contracts $ 550.0 Other current assets $ — Accrued compensation and other current liabilities $ 54.5 Other noncurrent assets — Other noncurrent liabilities 10.9 Foreign exchange contracts (1) 48.6 Other current assets 5.9 Accrued compensation and other current liabilities 0.6 Other noncurrent assets 7.8 Other noncurrent liabilities 3.3 Total $ 598.6 $ 13.7 $ 69.3 ( 1 ) Included within foreign exchange contracts at September 30, 2021 were $365.8 million of call options, offset with $365.8 million of put options, and $12.1 million of buy forwards, offset with $34.6 million of sell forwards. Included within foreign exchange contracts at December 31, 2020 were $354.7 million of call options, offset with $354.7 million of put options, and $9.0 million of buy forwards, offset with $57.6 million of sell forwards. The effect of cash flow hedge accounting on accumulated other comprehensive loss for the three and nine-month periods ended September 30, 2021 and 2020 were as follows (in millions): Amount of Amount of Gain (Loss) Loss Recognized Amount of Reclassified in Earnings Gain (Loss) from Related to Recognized in Accumulated Amount Accumulated Other Excluded Other Comprehensive from Comprehensive Loss into Effectiveness Statement of Earnings Instrument Loss (1) Earnings Testing Classification Three-month period ended September 30, 2021 Interest rate contracts $ 11.2 $ (0.3 ) $ — Interest expense Foreign exchange contracts (5.6 ) (0.4 ) 0.2 Commission revenue (0.1 ) 0.2 Compensation expense (0.2 ) 0.1 Operating expense Total $ 5.6 $ (1.0 ) $ 0.5 Three-month period ended September 30, 2020 Interest rate contracts $ 3.4 $ (0.3 ) $ — Interest expense Foreign exchange contracts 7.0 (0.4 ) (0.1 ) Commission revenue (0.4 ) 0.4 Compensation expense (0.3 ) 0.4 Operating expense Total $ 10.4 $ (1.4 ) $ 0.7 Nine-month period ended September 30, 2021 Interest rate contracts $ 45.3 $ (0.9 ) $ — Interest expense Foreign exchange contracts (11.7 ) (2.7 ) — Commission revenue (0.6 ) 0.8 Compensation expense (0.5 ) 0.6 Operating expense Total $ 33.6 $ (4.7 ) $ 1.4 Nine-month period ended September 30, 2020 Interest rate contracts $ (104.7 ) $ (0.9 ) $ — Interest expense Foreign exchange contracts (10.3 ) (0.9 ) (0.4 ) Commission revenue (1.2 ) 1.0 Compensation expense (0.9 ) 0.8 Operating expense Total $ (115.0 ) $ (3.9 ) $ 1.4 (1) For the three and nine-month periods ended September 30, 2021, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of zero and $1.2 million, respectively. For the three and nine-month periods ended September 30, 2020, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of $0.4 million and $0.8 million, respectively. We estimate that approximately $8.1 million of pretax loss currently included within accumulated other comprehensive loss will be reclassified into earnings in the next twelve months.

Commitments, Contingencies and

Commitments, Contingencies and Off-Balance Sheet Arrangements	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments, Contingencies and Off-Balance Sheet Arrangements	14. In connection with our investing and operating activities, we have entered into certain contractual obligations and commitments. Our future minimum cash payments, including interest, associated with our contractual obligations pursuant to the Senior Notes, Note purchase agreements, Credit Agreement, Premium Financing Debt Facility, operating leases and purchase obligations at September 30, 2021 were as follows (in millions): Payments Due by Period Contractual Obligations 2021 2022 2023 2024 2025 Thereafter Total Senior Notes $ — $ — $ — $ — $ — $ 850.0 $ 850.0 Note purchase agreements — 200.0 250.0 475.0 200.0 3,323.0 4,448.0 Credit Agreement — — — — — — — Premium Financing Debt Facility 234.1 — — — — — 234.1 Interest on debt 59.9 217.7 208.7 192.0 177.4 1,406.9 2,262.6 Total debt obligations 294.0 417.7 458.7 667.0 377.4 5,579.9 7,794.7 Operating lease obligations 21.8 102.0 81.9 59.5 45.5 88.2 398.9 Less sublease arrangements (0.3 ) (0.8 ) (0.6 ) (0.3 ) (0.2 ) (0.5 ) (2.7 ) Outstanding purchase obligations 33.5 68.9 47.3 30.7 22.9 37.4 240.7 Total contractual obligations $ 349.0 $ 587.8 $ 587.3 $ 756.9 $ 445.6 $ 5,705.0 $ 8,431.6 The amounts presented in the table above may not necessarily reflect our actual future cash funding requirements, because the actual timing of the future payments made may vary from the stated contractual obligation. Senior Notes, Note Purchase Agreements, Credit Agreement and Premium Financing Debt Facility - See Note 7 to these unaudited consolidated financial statements for a summary of the amounts outstanding under the Senior Notes, Note purchase agreements, the Credit Agreement and Premium Financing Debt Facility. On July 29, 2021, we exercised the special option redemption feature for the 2031 Senior Notes (which is described in further detail in Note 7). These notes were redeemed on August 13, 2021. Operating Lease Obligations - Our corporate segment’s executive offices and certain subsidiary and branch facilities of our brokerage and risk management segments are located in a building we own at 2850 Golf Road, Rolling Meadows, Illinois, where we have approximately 360,000 square feet of space and will accommodate approximately 2,000 employees at peak pre-pandemic capacity We generally operate in leased premises at our other locations. Certain of these leases have options permitting renewals for additional periods. In addition to minimum fixed rentals, a number of leases contain annual escalation clauses which are generally related to increases in an inflation index. We have leased certain office space to several non-affiliated tenants under operating sublease arrangements. In the normal course of business, we expect that certain of these leases will not be renewed or replaced. We adjust charges for real estate taxes and common area maintenance annually based on actual expenses, and we recognize the related revenues in the year in which the expenses are incurred. These amounts are not included in the minimum future rentals to be received in the contractual obligations table above. Outstanding Purchase Obligations - We typically do not have a material amount of outstanding purchase obligations at any point in time. The amount disclosed in the contractual obligations table above represents the aggregate amount of unrecorded purchase obligations that we had outstanding at September 30, 2021. These obligations represent agreements to purchase goods or services that were executed in the normal course of business. Off-Balance Sheet Commitments Our total unrecorded commitments associated with outstanding letters of credit, financial guarantees and funding commitments as of September 30, 2021 were as follows (in millions): Total Amount of Commitment Expiration by Period Amounts Off-Balance Sheet Commitments 2021 2022 2023 2024 2025 Thereafter Committed Letters of credit $ — $ — $ — $ — $ — $ 15.1 $ 15.1 Financial guarantees 0.3 1.4 1.5 2.3 0.2 0.2 5.9 Funding commitments 0.5 — — — — — 0.5 Total commitments $ 0.8 $ 1.4 $ 1.5 $ 2.3 $ 0.2 $ 15.3 $ 21.5 Since commitments may expire unused, the amounts presented in the table above do not necessarily reflect our actual future cash funding requirements. the letters of credit. All of the letters of credit represent multiple year commitments that have annual, automatic renewing provisions and are classified by the latest commitment date. Since January 1, 2002, we have acquired 602 companies, all of which were accounted for using the acquisition method for recording business combinations. Substantially all of the purchase agreements related to these acquisitions contain provisions for potential earnout obligations. For all of our acquisitions made in the period from 2017 to 2021 that contain potential earnout obligations, such obligations are measured at fair value as of the acquisition date and are included on that basis in the recorded purchase price consideration for the respective acquisition. The amounts recorded as earnout payables are primarily based upon estimated future potential operating results of the acquired entities over a two- to three-year period subsequent to the acquisition date. The aggregate amount of the maximum earnout obligations related to these acquisitions was $1,026.1 million, of which $578.7 million was recorded in our consolidated balance sheet as of September 30, 2021 based on the estimated fair value of the expected future payments to be made, of which approximately $560.5 million can be settled in cash or stock at our option and $18.2 million must be settled in cash. Off-Balance Sheet Debt Our unconsolidated investment portfolio includes investments in enterprises where our ownership interest is between 1% and 50%, in which management has determined that our level of influence and economic interest is not sufficient to require consolidation. As a result, these investments are accounted for under the equity method. None of these unconsolidated investments had any outstanding debt at September 30, 2021 or December 31, 2020, that was recourse to us At September 30, 2021, we had posted two letters of credit totaling $9.4 million, in the aggregate, related to our self‑insurance deductibles, for which we had a recorded liability of $18.7 million. We have an equity investment in a rent-a-captive facility, which we use as a placement facility for certain of our insurance brokerage operations. At September 30, 2021, we had posted six letters of credit totaling $4.3 million to allow certain of our captive operations to meet minimum statutory surplus requirements plus additional collateral related to premium and claim funds held in a fiduciary capacity, one letter of credit totaling $0.9 million for collateral related to claim funds held in a fiduciary capacity by a recent acquisition, and one letter of credit totaling $0.5 million as a security deposit for a 2015 acquisition’s lease. These letters of credit have never been drawn upon. Litigation, Regulatory and Taxation Matters - We are a defendant in various legal actions incidental to the nature of our business including but not limited to matters related to employment practices, alleged breaches of non‑compete or other restrictive covenants, theft of trade secrets, breaches of fiduciary duties and related causes of action. We are also periodically the subject of inquiries, investigations and reviews by regulatory and taxing authorities into various matters related to our business, including our operational, compliance and finance functions. Neither the outcomes of these matters nor their effect upon our business, financial condition or results of operations can be determined at this time. In July 2019, Midwest Energy Emissions Corp. and MES Inc. (which we refer to together as Midwest Energy) filed a patent infringement lawsuit in the United States District Court for the District of Delaware against us, Chem‑Mod LLC and numerous other related and unrelated parties. The complaint alleges that the named defendants’ infringe patents held exclusively by Midwest Energy and seeks unspecified damages and injunctive relief. The case is in discovery. We continue to defend this matter vigorously. Litigation is inherently uncertain and it is not possible for us to predict the ultimate outcome of this matter and the financial impact to us. We believe the probability of a material loss is remote. As previously disclosed, our IRC 831(b) (or “micro-captive”) advisory services business has been under audit by the IRS since 2013. Among other matters, the IRS is investigating whether we have been acting as a tax shelter promoter in connection with these operations. Additionally, the IRS is conducting a criminal investigation related to IRC 831(b) micro-captive underwriting enterprises. We have been advised that we are not a target of the criminal investigation. We are fully cooperating with both matters. While we are not able to reasonably estimate the amount of any potential loss in connection with these matters, we do not expect any such loss to be material. Contingent Liabilities - We purchase insurance to provide protection from errors and omissions ( which we refer to as E&O) claims that may arise during the ordinary course of business. C urrently we retain the first $ 15.0 million of every E&O claim up to $ 15.0 million . In addition, we retain, in aggregate: up to another $ 2.0 million between $ 15.0 million and $ 100.0 million, plus up to another $ 20.0 million between $ 100.0 million and $ 225.0 million, and up to another $ 10.0 million between $ 225.0 million a n d $ 365.0 million. We have historically maintained self-insurance reserves for the portion of our E&O exposure that is not insured. We periodically determine a range of possible reserve levels using actuarial techniques that rely heavily on projecting historical claim data into the future. Our E&O reserve in the September 30, 2021 consolidated balance sheet is above the lower end of the most recently determined actuarial range by $ 2.8 million and below the upper end of the actuarial range by $ 7.4 million. In addition to this E&O reserve, in the nine-month period ended September 30, 2021 , we established provision s for potential unusual pandemic related claim defense and other costs . We can make no assurances that the historical claim data used to project the current reserve levels will be indicative of future claim activity. Thus, the E&O reserve level and corresponding actuarial range could change in the future as more information becomes known, which could materially impact the amounts reported and disclosed herein . Tax-advantaged Investments No Longer Held - Between 1996 and 2007, we developed and then sold portions of our ownership in various energy related investments, many of which qualified for tax credits under IRC Section 29. We recorded tax benefits in connection with our ownership in these investments. At September 30, 2021, we had exposure on $108.0 million of previously earned tax credits. Under the TCJA, a portion of these previously earned tax credits were refunded in 2019 for tax year 2018, according to a specific formula. Under the Coronavirus Aid, Relief, and Economic Security Act (the CARES Act), which was passed on March 27, 2020, we accelerated the refund of all remaining credits on April 17, 2020, and the remaining credits were refunded to us in the second quarter of 2020. In 2004, 2007 and 2009, the IRS examined several of these investments and all examinations were closed without any changes being proposed by the IRS. However, any future adverse tax audits, administrative rulings or judicial decisions could disallow previously claimed tax credits Due to the contingent nature of this exposure and our related assessment of its likelihood, no reserve has been recorded in our September 30, 2021 consolidated balance sheet related to this exposure.

Supplemental Disclosures of Cas

Supplemental Disclosures of Cash Flow Information	9 Months Ended
	Sep. 30, 2021
Cash And Cash Equivalents [Abstract]
Supplemental Disclosures of Cash Flow Information	15. Nine-month period ended September 30, Supplemental disclosures of cash flow information (in millions): 2021 2020 Interest paid $ 150.4 $ 144.8 Income taxes paid, net 235.2 65.6 The following is a reconciliation of our end of period cash, cash equivalents and restricted cash balances as presented in the consolidated statement of cash flows for the nine-month periods ended September 30, 2021 and 2020 (in millions): September 30, 2021 2020 Cash and cash equivalents $ 2,735.1 $ 629.9 Restricted cash 3,277.2 2,844.9 Total cash, cash equivalents and restricted cash $ 6,012.3 $ 3,474.8 We have a qualified contributory savings and thrift 401(k) plan covering the majority of our domestic employees. For eligible employees who have met the plan’s age and service requirements to receive matching contributions, we historically have matched 100% of pre-tax and Roth elective deferrals up to a maximum of 5.0% of eligible compensation, subject to federal limits on plan contributions and not in excess of the maximum amount deductible for federal income tax purposes. Beginning with the match paid in 2021, the amount matched by the company will be discretionary and annually determined by management. Employees must be employed and eligible for the plan on the last day of the plan year to receive a matching contribution, subject to certain exceptions enumerated in the plan document. Matching contributions are subject to a five-year

Accumulated Other Comprehensive

Accumulated Other Comprehensive Loss	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Accumulated Other Comprehensive Loss	16. The after-tax components of our accumulated other comprehensive loss attributable to controlling interests consist of the following: Foreign Fair Value of Accumulated Pension Currency Derivative Comprehensive Liability Translation Investments Loss Balance as of December 31, 2020 $ (56.1 ) $ (491.1 ) $ (96.4 ) $ (643.6 ) Net change in period 1.7 (9.7 ) 28.4 20.4 Balance as of September 30, 2021 $ (54.4 ) $ (500.8 ) $ (68.0 ) $ (623.2 ) The foreign currency translation during the nine-month period ended September 30, 2021 primarily relates to the net impact of changes in the value of the local currencies relative to the U.S. dollar for our operations in the U.K., Australia, Canada, New Zealand, the Caribbean, India and other non-U.S. locations. During the nine-month periods ended September 30, 2021 and 2020, $4.3 million and $4.6 million, respectively, of expense related to the pension liability was reclassified from accumulated other comprehensive loss to compensation expense in the statement of earnings. During the nine-month periods ended September 30, 2021 and 2020, $4.7 million and $3.9 million of expense, respectively, related to the fair value of derivative investments, was reclassified from accumulated other comprehensive loss to the statement of earnings. During the nine-month periods ended September 30, 2021 and 2020, no amounts related to foreign currency translation were reclassified from accumulated other comprehensive loss to the statement of earnings.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Segment Information	17. We have three reportable segments: brokerage, risk management and corporate. The brokerage segment is primarily comprised of our retail and wholesale insurance brokerage operations. The segment generates revenues through The risk management segment provides contract claim settlement and administration services for enterprises and public entities that choose to self-insure some or all of their property/casualty coverages and for underwriting enterprises that choose to outsource some or all of their property/casualty claims departments. These operations also provide claims management, loss control consulting and insurance property appraisal services. Revenues are principally generated on a negotiated per-claim or per-service fee basis. Our risk management segment also provides risk management consulting services that are recognized as the services are delivered. The corporate segment manages Allocations of investment income and certain expenses are based on reasonable assumptions and estimates primarily using revenue, headcount and other information. We allocate the provision for income taxes to the brokerage and risk management segments using the local country statutory rates. Reported operating results by segment would change if different methods were applied. Financial information relating to our segments for the three and nine-month periods ended September 30, 2021 and 2020 is as follows (in millions): Three-month period ended September 30, Nine-month period ended September 30, 2021 2020 2021 2020 Brokerage Total revenues $ 1,499.7 $ 1,294.6 $ 4,500.1 $ 3,931.3 Earnings before income taxes $ 334.5 $ 276.4 $ 1,114.5 $ 935.0 Identifiable assets at September 30, 2021 and 2020 $ 21,184.7 $ 18,641.2 Risk Management Total revenues $ 280.6 $ 244.3 $ 815.0 $ 717.0 Earnings before income taxes $ 29.5 $ 25.3 $ 87.0 $ 64.1 Identifiable assets at September 30, 2021 and 2020 $ 1,079.7 $ 964.5 Corporate Total revenues $ 357.9 $ 310.2 $ 921.6 $ 651.7 Loss before income taxes $ (132.6 ) $ (92.2 ) $ (339.4 ) $ (261.9 ) Identifiable assets at September 30, 2021 and 2020 $ 4,651.4 $ 2,209.2 Total Total revenues $ 2,138.2 $ 1,849.1 $ 6,236.7 $ 5,300.0 Earnings before income taxes $ 231.4 $ 209.5 $ 862.1 $ 737.2 Identifiable assets at September 30, 2021 and 2020 $ 26,915.8 $ 21,814.9 Disaggregation of Revenue We disaggregate our revenue from contracts with clients by type and geographic location for each of our segments, as we believe it best depicts how the nature, amount, timing and uncertainty of our revenue and cash flows are affected by economic factors. Revenues by type and segment for the three-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 1,016.2 $ — $ — $ 1,016.2 Fees 353.8 247.9 — 601.7 Supplemental revenues 61.0 — — 61.0 Contingent revenues 43.7 — — 43.7 Investment income 20.7 0.1 — 20.8 Net gains on divestitures 4.3 — — 4.3 Revenues from clean coal activities — — 357.6 357.6 Other net revenues — — 0.3 0.3 Revenues before reimbursements 1,499.7 248.0 357.9 2,105.6 Reimbursements — 32.6 — 32.6 Total revenues $ 1,499.7 $ 280.6 $ 357.9 $ 2,138.2 Revenues by type and segment for the nine-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 3,118.7 $ — $ — $ 3,118.7 Fees 983.4 713.0 — 1,696.4 Supplemental revenues 183.0 — — 183.0 Contingent revenues 150.3 — — 150.3 Investment income 55.8 0.2 — 56.0 Net gains on divestitures 8.9 0.1 — 9.0 Revenues from clean coal activities — — 919.2 919.2 Other net revenues — — 2.4 2.4 Revenues before reimbursements 4,500.1 713.3 921.6 6,135.0 Reimbursements — 101.7 — 101.7 Total revenues $ 4,500.1 $ 815.0 $ 921.6 $ 6,236.7 Revenues by geographical location and segment for the three-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 1,000.3 $ 233.9 $ 357.9 $ 1,592.1 United Kingdom 276.5 12.1 — 288.6 Australia 62.4 29.6 — 92.0 Canada 68.1 1.6 — 69.7 New Zealand 41.0 3.4 — 44.4 Other foreign 51.4 — — 51.4 Total revenues $ 1,499.7 $ 280.6 $ 357.9 $ 2,138.2 Revenues by geographical location and segment for the nine-month period ended September 30, 2021 Risk Brokerage Management Corporate Total Revenues: United States $ 2,919.3 $ 672.9 $ 921.6 $ 4,513.8 United Kingdom 895.0 35.0 — 930.0 Australia 187.1 91.6 — 278.7 Canada 215.5 4.4 — 219.9 New Zealand 118.9 11.1 — 130.0 Other foreign 164.3 — — 164.3 Total revenues $ 4,500.1 $ 815.0 $ 921.6 $ 6,236.7 Revenues by type and segment for the three-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 889.9 $ — $ — $ 889.9 Fees 293.2 202.6 — 495.8 Supplemental revenues 54.7 — — 54.7 Contingent revenues 34.5 — — 34.5 Investment income 18.9 0.1 — 19.0 Net gains on divestitures 3.4 — — 3.4 Revenues from clean coal activities — — 310.8 310.8 Other net losses — — (0.6 ) (0.6 ) Revenues before reimbursements 1,294.6 202.7 310.2 1,807.5 Reimbursements — 41.6 — 41.6 Total revenues $ 1,294.6 $ 244.3 $ 310.2 $ 1,849.1 Revenues by type and segment for the nine-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 2,734.6 $ — $ — $ 2,734.6 Fees 858.1 604.7 — 1,462.8 Supplemental revenues 164.0 — — 164.0 Contingent revenues 117.0 — — 117.0 Investment income 53.0 0.6 — 53.6 Net gains on divestitures 4.6 — — 4.6 Revenues from clean coal activities — — 652.1 652.1 Other net losses — — (0.4 ) (0.4 ) Revenues before reimbursements 3,931.3 605.3 651.7 5,188.3 Reimbursements — 111.7 — 111.7 Total revenues $ 3,931.3 $ 717.0 $ 651.7 $ 5,300.0 Revenues by geographical location and segment for the three-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 866.6 $ 203.9 $ 310.2 $ 1,380.7 United Kingdom 230.8 9.8 — 240.6 Australia 56.6 26.2 — 82.8 Canada 57.1 1.4 — 58.5 New Zealand 37.0 3.0 — 40.0 Other foreign 46.5 — — 46.5 Total revenues $ 1,294.6 $ 244.3 $ 310.2 $ 1,849.1 Revenues by geographical location and segment for the nine-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 2,613.3 $ 602.2 $ 651.7 $ 3,867.2 United Kingdom 746.2 29.4 — 775.6 Australia 156.6 72.5 — 229.1 Canada 171.5 3.9 — 175.4 New Zealand 103.0 9.0 — 112.0 Other foreign 140.7 — — 140.7 Total revenues $ 3,931.3 $ 717.0 $ 651.7 $ 5,300.0

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Nature of Operations and Basis of Presentation	Nature of Operations and Basis of Presentation Arthur J. Gallagher & Co. and its subsidiaries, collectively referred to herein as we, our, us or the company, provide insurance brokerage, consulting and third party claims settlement and administration services to both domestic and international entities. We have three reportable segments: brokerage, risk management and corporate. Our brokers, agents and administrators act as intermediaries between underwriting enterprises and our clients. Our brokerage segment operations provide brokerage and consulting services to companies and entities of all types, including commercial, not-for-profit, public entities, and, to a lesser extent, individuals, in the areas of insurance placement, risk of loss management, and management of employer sponsored benefit programs. Our risk management segment operations provide contract claim settlement, claim administration, loss control services and risk management consulting for commercial, not-for-profit, captive and public entities, and various other organizations that choose to self-insure property/casualty coverages or choose to use a third‑party claims management organization rather than the claim services provided by underwriting enterprises. The corporate segment reports the financial information related to our debt and other corporate costs, clean energy investments, external acquisition‑related expenses and the impact of foreign currency translation. Clean energy investments consist of our investments in limited liability companies that own 35 commercial clean coal production facilities that can produce refined coal using Chem-Mod LLC’s proprietary technologies. We believe these operations produce refined coal that qualifies for tax credits under IRC Section 45. We do not assume underwriting risk on a net basis, other than with respect to de minimis amounts necessary to provide minimum or regulatory capital to organize captives, pools, specialized underwriters or risk-retention groups. Rather, capital necessary for covering losses is provided by underwriting enterprises. Investment income and other revenues are primarily generated from our premium financing operations, our invested cash and restricted cash we hold on behalf of our clients, as well as clean energy investments. In addition, our share of the net earnings related to partially owned entities that are accounted for using the equity method is included in investment income. We are headquartered in Rolling Meadows, Illinois, have operations in 57 countries and offer client-service capabilities in more than 150 countries globally through a network of correspondent insurance brokers and consultants. We have prepared the accompanying unaudited consolidated financial statements pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and footnote disclosures normally included in annual financial statements have been omitted pursuant to such rules and regulations. The unaudited consolidated financial statements included herein are, in the opinion of management, prepared on a basis consistent with our audited consolidated financial statements for the year ended December 31, 2020, and include all normal recurring adjustments necessary for a fair presentation of the information set forth. The quarterly results of operations are not necessarily indicative of the results of operations to be reported for subsequent quarters or the full year. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2020. In the preparation of our unaudited consolidated financial statements as of September 30, 2021, management evaluated all material subsequent events or transactions that occurred after the balance sheet date through the date on which the financial statements were issued, for potential recognition and/or disclosure therein.
Use of Estimates	Use of Estimates The preparation of our consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. These accounting principles require us to make estimates and assumptions that affect the reported amounts of assets and liabilities and revenues and expenses, and the disclosure of contingent assets and liabilities at the date of our consolidated financial statements. We periodically evaluate our estimates and assumptions, including those relating to the valuation of goodwill and other intangible assets, right-of-use assets, investments (including our IRC Section 45 investments), income taxes, revenue recognition, deferred costs, stock-based compensation, claims handling obligations, retirement plans, litigation and contingencies. We base our estimates on historical experience and various assumptions that we believe to be reasonable based on specific circumstances. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed in the notes herein.

Business Combinations (Tables)

Business Combinations (Tables)	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Acquisition Method for Recording Business Combinations	During the nine-month period ended September 30, 2021, we acquired substantially all of the net assets of the following firms in exchange for our common stock and/or cash. These acquisitions have been accounted for using the acquisition method for recording business combinations (in millions, except share data): Total Maximum Common Common Recorded Recorded Potential Name and Effective Shares Share Accrued Escrow Earnout Purchase Earnout Date of Acquisition Issued Value Cash Paid Liability Deposited Payable Price Payable (000s) Atlas General Holdings, LLC January 1, 2021 (AGH) — $ — $ 95.2 $ — $ 5.4 $ 8.7 $ 109.3 $ 35.0 Bollington Wilson Group February 5, 2021 (BWG) — — 328.0 — 1.4 — 329.4 — LDJ American Online Benefits Group, LLC May 1, 2021 (LDJ) — — 38.2 — 7.1 14.8 60.1 20.0 Sixteen other acquisitions completed in 2021 337.0 44.4 157.3 13.1 18.9 34.1 267.8 70.2 337.0 $ 44.4 $ 618.7 $ 13.1 $ 32.8 $ 57.6 $ 766.6 $ 125.2
Summary of Estimated Fair Values of Net Assets Acquired	The following is a summary of the estimated fair values of the net assets acquired at the date of each acquisition made in the nine‑month period ended September 30, 2021 (in millions): Sixteen Other AGH BWG LDJ Acquisitions Total Cash and restricted cash $ 32.8 $ 24.9 $ 0.7 $ 11.0 $ 69.4 Other current assets 112.0 15.3 15.8 51.5 194.6 Fixed assets 7.4 3.8 — 0.2 11.4 Noncurrent assets 1.2 5.3 0.2 6.2 12.9 Goodwill 45.9 257.1 25.7 124.8 453.5 Expiration lists 39.7 81.6 25.4 133.2 279.9 Non-compete agreements 0.3 1.2 0.2 2.1 3.8 Trade names — 3.4 0.2 1.1 4.7 Total assets acquired 239.3 392.6 68.2 330.1 1,030.2 Current liabilities 129.8 39.6 8.0 49.9 227.3 Noncurrent liabilities 0.2 23.6 0.1 12.4 36.3 Total liabilities assumed 130.0 63.2 8.1 62.3 263.6 Total net assets acquired $ 109.3 $ 329.4 $ 60.1 $ 267.8 $ 766.6
Summary of Unaudited Pro Forma Historical Results	The following is a summary of the unaudited pro forma historical results, as if these entities had been acquired at January 1, 2020 (in millions, except per share data): Three-month period ended Nine-month period ended September 30, September 30, 2021 2020 2021 2020 Total revenues $ 2,140.7 $ 1,899.6 $ 6,278.4 $ 5,435.7 Net earnings attributable to controlling interests 225.0 181.6 801.9 685.9 Basic net earnings per share 1.09 0.94 3.99 3.60 Diluted net earnings per share 1.06 0.92 3.90 3.52

Contracts with Customers (Table

Contracts with Customers (Tables)	9 Months Ended
	Sep. 30, 2021
Revenue From Contract With Customer [Abstract]
Summary of Unbilled Receivables, Contract Assets and Contract Liabilities from Contracts with Customers	Information about unbilled receivables, contract assets and contract liabilities from contracts with customers is as follows (in millions): September 30 2021 December 31, 2020 Unbilled receivables $ 810.6 $ 603.1 Deferred contract costs 76.3 102.0 Deferred revenue 588.2 541.3
Summary of Changes in Deferred Revenue Balances	Significant changes in the deferred revenue balances, which include foreign currency translation adjustments, during the period are as follows (in millions): Risk Brokerage Management Total Deferred revenue at December 31, 2020 $ 354.7 $ 186.6 $ 541.3 Incremental deferred revenue 283.3 77.4 360.7 Revenue recognized during the nine-month period ended September 30, 2021 included in deferred revenue at December 31, 2020 (256.5 ) (76.8 ) (333.3 ) Net change in collected billings/deposits received from customers 1.5 7.3 8.8 Impact of change in foreign exchange rates 1.0 — 1.0 Deferred revenue recognized from business acquisitions 9.7 — 9.7 Deferred revenue at September 30, 2021 $ 393.7 $ 194.5 $ 588.2
Summary of Expected Revenue Related to Performance Obligations	The estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period is as follows (in millions): Brokerage Risk Management Total 2021 (remaining three months) $ 235.6 $ 89.4 $ 325.0 2022 135.8 54.6 190.4 2023 19.3 23.4 42.7 2024 1.5 10.6 12.1 2025 0.8 5.9 6.7 Thereafter 0.7 10.6 11.3 Total $ 393.7 $ 194.5 $ 588.2

Other Financial Data (Tables)

Other Financial Data (Tables)	9 Months Ended
	Sep. 30, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Summary of Major Classes of Other Current Assets	Major classes of other current assets consist of the following (in millions): September 30, 2021 December 31, 2020 Premium finance advances and loans $ 511.8 $ 442.7 Accrued supplemental, direct bill and other receivables 546.2 361.7 Refined coal production related receivables 108.5 95.4 Deferred contract costs 76.3 102.0 Prepaid expenses 126.7 112.1 Total other current assets $ 1,369.5 $ 1,113.9

Intangible Assets (Tables)

Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations	The carrying amount of goodwill at September 30, 2021 and December 31, 2020 allocated by domestic and foreign operations is as follows (in millions): Brokerage Risk Management Corporate Total At September 30, 2021 United States $ 3,568.9 $ 64.8 $ — $ 3,633.7 United Kingdom 1,606.4 15.4 — 1,621.8 Canada 523.0 — — 523.0 Australia 450.0 11.0 — 461.0 New Zealand 220.5 10.6 — 231.1 Other foreign 154.0 — 2.9 156.9 Total goodwill $ 6,522.8 $ 101.8 $ 2.9 $ 6,627.5 At December 31, 2020 United States $ 3,399.8 $ 33.2 $ — $ 3,433.0 United Kingdom 1,328.3 15.1 — 1,343.4 Canada 492.9 — — 492.9 Australia 462.1 11.5 — 473.6 New Zealand 221.9 10.7 — 232.6 Other foreign 148.6 — 2.9 151.5 Total goodwill $ 6,053.6 $ 70.5 $ 2.9 $ 6,127.0
Changes in Carrying Amount of Goodwill	The changes in the carrying amount of goodwill for the nine-month period ended September 30, 2021 are as follows (in millions): Brokerage Risk Management Corporate Total Balance as of December 31, 2020 $ 6,053.6 $ 70.5 $ 2.9 $ 6,127.0 Goodwill acquired during the period 421.9 31.6 — 453.5 Goodwill adjustments due to appraisals and other acquisition adjustments 50.5 — — 50.5 Goodwill written-off related to sales of business and impairment (1.0 ) — — (1.0 ) Foreign currency translation adjustments during the period (2.2 ) (0.3 ) — (2.5 ) Balance as of September 30, 2021 $ 6,522.8 $ 101.8 $ 2.9 $ 6,627.5
Major Classes of Amortizable Intangible Assets	Major classes of amortizable intangible assets at September 30, 2021 and December 31, 2020 consist of the following (in millions): September 30, December 31, 2021 2020 Expiration lists $ 4,997.2 $ 4,753.2 Accumulated amortization - expiration lists (2,727.5 ) (2,436.7 ) 2,269.7 2,316.5 Non-compete agreements 73.5 75.9 Accumulated amortization - non-compete agreements (61.9 ) (57.8 ) 11.6 18.1 Trade names 105.5 107.8 Accumulated amortization - trade names (47.8 ) (42.5 ) 57.7 65.3 Net amortizable assets $ 2,339.0 $ 2,399.9
Estimated Aggregate Amortization Expense	Estimated aggregate amortization expense for each of the next five years and thereafter is as follows (in millions): 2021 (remaining three months) $ 96.0 2022 367.7 2023 342.4 2024 306.1 2025 266.2 Thereafter 960.6 Total $ 2,339.0

Credit and Other Debt Agreeme_2

Credit and Other Debt Agreements (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Summary of Corporate and Other Debt	7. Credit and Other Debt Agreements The following is a summary of our corporate and other debt (in millions): September 30, December 31, 2021 2020 Senior Notes: Semi-annual payments of interest, fixed rate of 3.50%, balloon due May 20, 2051 850.0 — Total Senior Notes 850.0 — Note Purchase Agreements: Semi-annual payments of interest, fixed rate of 5.18%, balloon due February 10, 2021 — 75.0 Semi-annual payments of interest, fixed rate of 3.69%, balloon due June 14, 2022 200.0 200.0 Semi-annual payments of interest, fixed rate of 5.49%, balloon due February 10, 2023 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.13%, balloon due June 24, 2023 200.0 200.0 Semi-annual payments of interest, fixed rate of 4.72%, balloon due February 13, 2024 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.58%, balloon due February 27, 2024 325.0 325.0 Quarterly payments of interest, floating rate of 90 day LIBOR plus 1.40%, balloon due June 13, 2024 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.31%, balloon due June 24, 2025 200.0 200.0 Semi-annual payments of interest, fixed rate of 4.85%, balloon due February 13, 2026 140.0 140.0 Semi-annual payments of interest, fixed rate of 4.73%, balloon due February 27, 2026 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.40%, balloon due June 2, 2026 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.36%, balloon due June 24, 2026 150.0 150.0 Semi-annual payments of interest, fixed rate of 3.75%, balloon due January 30, 2027 30.0 30.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due June 27, 2027 125.0 125.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due August 2, 2027 125.0 125.0 Semi-annual payments of interest, fixed rate of 4.14%, balloon due August 4, 2027 98.0 98.0 Semi-annual payments of interest, fixed rate of 3.46%, balloon due December 1, 2027 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.55%, balloon due June 2, 2028 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due June 13, 2028 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.04%, balloon due February 13, 2029 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.98%, balloon due February 27, 2029 100.0 100.0 Semi-annual payments of interest, fixed rate of 4.19%, balloon due June 27, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 4.19%, balloon due August 2, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 3.48%, balloon due December 2, 2029 50.0 50.0 Semi-annual payments of interest, fixed rate of 3.99%, balloon due January 30, 2030 341.0 341.0 Semi-annual payments of interest, fixed rate of 4.44%, balloon due June 13, 2030 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.14%, balloon due March 13, 2031 180.0 180.0 Semi-annual payments of interest, fixed rate of 4.70%, balloon due June 2, 2031 25.0 25.0 Semi-annual payments of interest, fixed rate of 4.09%, balloon due January 30, 2032 69.0 69.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due June 27, 2032 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.34%, balloon due August 2, 2032 75.0 75.0 Semi-annual payments of interest, fixed rate of 4.59%, balloon due June 13, 2033 125.0 125.0 Semi-annual payments of interest, fixed rate of 5.29%, balloon due March 13, 2034 40.0 40.0 Semi-annual payments of interest, fixed rate of 4.48%, balloon due June 12, 2034 175.0 175.0 Semi-annual payments of interest, fixed rate of 4.24%, balloon due January 30, 2035 79.0 79.0 Semi-annual payments of interest, fixed rate of 2.44%, balloon due February 10, 2036 100.0 — Semi-annual payments of interest, fixed rate of 2.46%, balloon due May 5, 2036 75.0 — Semi-annual payments of interest, fixed rate of 4.69%, balloon due June 13, 2038 75.0 75.0 Semi-annual payments of interest, fixed rate of 5.45%, balloon due March 13, 2039 40.0 40.0 Semi-annual payments of interest, fixed rate of 4.49%, balloon due January 30, 2040 56.0 56.0 Total Note Purchase Agreements 4,448.0 4,348.0 Credit Agreement: Periodic payments of interest and principal, prime or LIBOR plus up to 1.45%, expires June 7, 2024 — — Premium Financing Debt Facility - expires September 15, 2023: Facility B AUD denominated tranche, interbank rates plus 1.200% 211.9 193.1 NZD denominated tranche, interbank rates plus 1.650% — — Facility C and D AUD denominated tranche, interbank rates plus 0.680% 11.8 — NZD denominated tranche, interbank rates plus 0.940% 10.4 10.5 Total Premium Financing Debt Facility 234.1 203.6 Total corporate and other debt 5,532.1 4,551.6 Less unamortized debt acquisition costs on Senior Notes and Note Purchase Agreements (15.6 ) (7.0 ) Less unamortized discount on Bonds Payable (10.0 ) — Net corporate and other debt $ 5,506.5 $ 4,544.6

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net EPS	The following table sets forth the computation of basic and diluted net earnings per share (in millions, except per share data): Three-month period ended Nine-month period ended September 30, September 30, 2021 2020 2021 2020 Net earnings attributable to controlling interests $ 225.1 $ 176.6 $ 797.4 $ 676.6 Weighted average number of common shares outstanding 207.0 191.9 201.0 190.4 Dilutive effect of stock options using the treasury stock method 4.5 4.4 4.5 4.1 Weighted average number of common and common equivalent shares outstanding 211.5 196.3 205.5 194.5 Basic net earnings per share $ 1.09 $ 0.92 $ 3.97 $ 3.55 Diluted net earnings per share $ 1.06 $ 0.90 $ 3.88 $ 3.48

Stock Option Plans (Tables)

Stock Option Plans (Tables)	9 Months Ended
	Sep. 30, 2021
Text Block [Abstract]
Black-Scholes Option Pricing Model with Weighted Average	For purposes of expense recognition, the estimated fair values of the stock option grants are amortized to expense over the options’ vesting period. We estimated the fair value of stock options at the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions: 2021 2020 Expected dividend yield 1.5 % 2.1 % Expected risk-free interest rate 0.9 % 0.7 % Volatility 22.9 % 17.3 % Expected life (in years) 5.4 5.4
Stock Option Activity and Related Information	The following is a summary of our stock option activity and related information for 2021 (in millions, except exercise price and year data): Nine-month period ended September 30, 2021 Weighted Average Weighted Remaining Shares Average Contractual Aggregate Under Exercise Term Intrinsic Option Price (in years) Value Beginning balance 7.5 $ 65.09 Granted 1.6 127.90 Exercised (1.1 ) 49.30 Forfeited or canceled (0.1 ) 86.21 Ending balance 7.9 $ 80.17 4.05 $ 539.1 Exercisable at end of period 2.3 $ 51.05 1.76 $ 228.0 Ending unvested and expected to vest 5.1 $ 91.53 4.97 $ 293.8
Other Information Regarding Stock Options Outstanding and Exercisable	Other information regarding stock options outstanding and exercisable at September 30, 2021 is summarized as follows (in millions, except exercise price and year data): Options Outstanding Options Exercisable Weighted Average Remaining Weighted Weighted Contractual Average Average Number Term Exercise Number Exercise Range of Exercise Prices Outstanding (in years) Price Exercisable Price $ 43.71 — $ 43.71 1.0 $ 1.46 $ 43.71 0.9 43.71 46.17 — 46.17 0.5 0.44 46.17 0.5 46.17 49.55 — 56.86 1.1 2.46 56.83 0.6 56.81 70.74 70.74 1.0 3.46 70.74 0.3 70.74 79.59 — 79.59 1.2 4.45 79.59 — 79.59 86.17 — 86.17 1.5 5.45 86.17 — 86.17 127.90 — 127.90 1.6 6.46 127.90 — — $ 43.71 — $ 127.90 7.9 4.05 $ 80.17 2.3 $ 51.05

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2021
Equity Method Investments And Joint Ventures [Abstract]
Investments Reported in Other Current and Non-Current Assets	The following is a summary of our investments included in other noncurrent assets in the consolidated balance sheet and the related funding commitments (in millions): September 30, 2021 December 31, 2020 Funding Assets Commitments Assets Chem-Mod LLC $ 4.0 $ — $ 4.0 Chem-Mod International LLC 2.0 — 2.0 Clean-coal investments: Non-controlling interest in a limited liability company that owns one 2011 Era Clean Coal Plant — — 0.1 Controlling interest in limited liability companies that own twenty 2011 Era Clean Coal Plants 2.6 — 12.4 Other investments 5.5 0.5 3.9 Total investments $ 14.1 $ 0.5 $ 22.4

Derivatives and Hedging Activ_2

Derivatives and Hedging Activity (Tables)	9 Months Ended
	Sep. 30, 2021
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Summary of Notional and Fair Values of Derivative Instruments	The notional and fair values of derivatives designated as hedging instruments are as follows at September 30, 2021 and December 31, 2020 (in millions): Derivative Assets Derivative Liabilities Notional Balance Sheet Fair Balance Sheet Fair Instrument Amount Classification Value Classification Value At September 30, 2021 Interest rate contracts $ 1,000.0 Other current assets $ 5.9 Accrued compensation and other current liabilities $ 0.3 Other noncurrent assets 5.9 Other noncurrent liabilities 4.2 Foreign exchange contracts (1) 22.5 Other current assets 4.5 Accrued compensation and other current liabilities 0.9 Other noncurrent assets 1.9 Other noncurrent liabilities 4.4 Total $ 1,022.5 $ 18.2 $ 9.8 At December 31, 2020 Interest rate contracts $ 550.0 Other current assets $ — Accrued compensation and other current liabilities $ 54.5 Other noncurrent assets — Other noncurrent liabilities 10.9 Foreign exchange contracts (1) 48.6 Other current assets 5.9 Accrued compensation and other current liabilities 0.6 Other noncurrent assets 7.8 Other noncurrent liabilities 3.3 Total $ 598.6 $ 13.7 $ 69.3 ( 1 ) Included within foreign exchange contracts at September 30, 2021 were $365.8 million of call options, offset with $365.8 million of put options, and $12.1 million of buy forwards, offset with $34.6 million of sell forwards. Included within foreign exchange contracts at December 31, 2020 were $354.7 million of call options, offset with $354.7 million of put options, and $9.0 million of buy forwards, offset with $57.6 million of sell forwards.
Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss	The effect of cash flow hedge accounting on accumulated other comprehensive loss for the three and nine-month periods ended September 30, 2021 and 2020 were as follows (in millions): Amount of Amount of Gain (Loss) Loss Recognized Amount of Reclassified in Earnings Gain (Loss) from Related to Recognized in Accumulated Amount Accumulated Other Excluded Other Comprehensive from Comprehensive Loss into Effectiveness Statement of Earnings Instrument Loss (1) Earnings Testing Classification Three-month period ended September 30, 2021 Interest rate contracts $ 11.2 $ (0.3 ) $ — Interest expense Foreign exchange contracts (5.6 ) (0.4 ) 0.2 Commission revenue (0.1 ) 0.2 Compensation expense (0.2 ) 0.1 Operating expense Total $ 5.6 $ (1.0 ) $ 0.5 Three-month period ended September 30, 2020 Interest rate contracts $ 3.4 $ (0.3 ) $ — Interest expense Foreign exchange contracts 7.0 (0.4 ) (0.1 ) Commission revenue (0.4 ) 0.4 Compensation expense (0.3 ) 0.4 Operating expense Total $ 10.4 $ (1.4 ) $ 0.7 Nine-month period ended September 30, 2021 Interest rate contracts $ 45.3 $ (0.9 ) $ — Interest expense Foreign exchange contracts (11.7 ) (2.7 ) — Commission revenue (0.6 ) 0.8 Compensation expense (0.5 ) 0.6 Operating expense Total $ 33.6 $ (4.7 ) $ 1.4 Nine-month period ended September 30, 2020 Interest rate contracts $ (104.7 ) $ (0.9 ) $ — Interest expense Foreign exchange contracts (10.3 ) (0.9 ) (0.4 ) Commission revenue (1.2 ) 1.0 Compensation expense (0.9 ) 0.8 Operating expense Total $ (115.0 ) $ (3.9 ) $ 1.4 (1) For the three and nine-month periods ended September 30, 2021, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of zero and $1.2 million, respectively. For the three and nine-month periods ended September 30, 2020, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of $0.4 million and $0.8 million, respectively.

Commitments, Contingencies an_2

Commitments, Contingencies and Off-Balance Sheet Arrangements (Tables)	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Contractual Obligations	Our future minimum cash payments, including interest, associated with our contractual obligations pursuant to the Senior Notes, Note purchase agreements, Credit Agreement, Premium Financing Debt Facility, operating leases and purchase obligations at September 30, 2021 were as follows (in millions): Payments Due by Period Contractual Obligations 2021 2022 2023 2024 2025 Thereafter Total Senior Notes $ — $ — $ — $ — $ — $ 850.0 $ 850.0 Note purchase agreements — 200.0 250.0 475.0 200.0 3,323.0 4,448.0 Credit Agreement — — — — — — — Premium Financing Debt Facility 234.1 — — — — — 234.1 Interest on debt 59.9 217.7 208.7 192.0 177.4 1,406.9 2,262.6 Total debt obligations 294.0 417.7 458.7 667.0 377.4 5,579.9 7,794.7 Operating lease obligations 21.8 102.0 81.9 59.5 45.5 88.2 398.9 Less sublease arrangements (0.3 ) (0.8 ) (0.6 ) (0.3 ) (0.2 ) (0.5 ) (2.7 ) Outstanding purchase obligations 33.5 68.9 47.3 30.7 22.9 37.4 240.7 Total contractual obligations $ 349.0 $ 587.8 $ 587.3 $ 756.9 $ 445.6 $ 5,705.0 $ 8,431.6
Off-Balance Sheet Commitments	Off-Balance Sheet Commitments Our total unrecorded commitments associated with outstanding letters of credit, financial guarantees and funding commitments as of September 30, 2021 were as follows (in millions): Total Amount of Commitment Expiration by Period Amounts Off-Balance Sheet Commitments 2021 2022 2023 2024 2025 Thereafter Committed Letters of credit $ — $ — $ — $ — $ — $ 15.1 $ 15.1 Financial guarantees 0.3 1.4 1.5 2.3 0.2 0.2 5.9 Funding commitments 0.5 — — — — — 0.5 Total commitments $ 0.8 $ 1.4 $ 1.5 $ 2.3 $ 0.2 $ 15.3 $ 21.5

Supplemental Disclosures of C_2

Supplemental Disclosures of Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2021
Cash And Cash Equivalents [Abstract]
Supplemental disclosures of cash flow information	Nine-month period ended September 30, Supplemental disclosures of cash flow information (in millions): 2021 2020 Interest paid $ 150.4 $ 144.8 Income taxes paid, net 235.2 65.6
Summary of Cash, Cash Equivalents and Restricted Cash	The following is a reconciliation of our end of period cash, cash equivalents and restricted cash balances as presented in the consolidated statement of cash flows for the nine-month periods ended September 30, 2021 and 2020 (in millions): September 30, 2021 2020 Cash and cash equivalents $ 2,735.1 $ 629.9 Restricted cash 3,277.2 2,844.9 Total cash, cash equivalents and restricted cash $ 6,012.3 $ 3,474.8

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Loss (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Loss Attributable to Controlling Interests	The after-tax components of our accumulated other comprehensive loss attributable to controlling interests consist of the following: Foreign Fair Value of Accumulated Pension Currency Derivative Comprehensive Liability Translation Investments Loss Balance as of December 31, 2020 $ (56.1 ) $ (491.1 ) $ (96.4 ) $ (643.6 ) Net change in period 1.7 (9.7 ) 28.4 20.4 Balance as of September 30, 2021 $ (54.4 ) $ (500.8 ) $ (68.0 ) $ (623.2 )

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Schedule of Segment Reporting Information by Segment	Financial information relating to our segments for the three and nine-month periods ended September 30, 2021 and 2020 is as follows (in millions): Three-month period ended September 30, Nine-month period ended September 30, 2021 2020 2021 2020 Brokerage Total revenues $ 1,499.7 $ 1,294.6 $ 4,500.1 $ 3,931.3 Earnings before income taxes $ 334.5 $ 276.4 $ 1,114.5 $ 935.0 Identifiable assets at September 30, 2021 and 2020 $ 21,184.7 $ 18,641.2 Risk Management Total revenues $ 280.6 $ 244.3 $ 815.0 $ 717.0 Earnings before income taxes $ 29.5 $ 25.3 $ 87.0 $ 64.1 Identifiable assets at September 30, 2021 and 2020 $ 1,079.7 $ 964.5 Corporate Total revenues $ 357.9 $ 310.2 $ 921.6 $ 651.7 Loss before income taxes $ (132.6 ) $ (92.2 ) $ (339.4 ) $ (261.9 ) Identifiable assets at September 30, 2021 and 2020 $ 4,651.4 $ 2,209.2 Total Total revenues $ 2,138.2 $ 1,849.1 $ 6,236.7 $ 5,300.0 Earnings before income taxes $ 231.4 $ 209.5 $ 862.1 $ 737.2 Identifiable assets at September 30, 2021 and 2020 $ 26,915.8 $ 21,814.9
Summary of Revenues by Type and Segment	Revenues by type and segment for the three-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 1,016.2 $ — $ — $ 1,016.2 Fees 353.8 247.9 — 601.7 Supplemental revenues 61.0 — — 61.0 Contingent revenues 43.7 — — 43.7 Investment income 20.7 0.1 — 20.8 Net gains on divestitures 4.3 — — 4.3 Revenues from clean coal activities — — 357.6 357.6 Other net revenues — — 0.3 0.3 Revenues before reimbursements 1,499.7 248.0 357.9 2,105.6 Reimbursements — 32.6 — 32.6 Total revenues $ 1,499.7 $ 280.6 $ 357.9 $ 2,138.2 Revenues by type and segment for the nine-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 3,118.7 $ — $ — $ 3,118.7 Fees 983.4 713.0 — 1,696.4 Supplemental revenues 183.0 — — 183.0 Contingent revenues 150.3 — — 150.3 Investment income 55.8 0.2 — 56.0 Net gains on divestitures 8.9 0.1 — 9.0 Revenues from clean coal activities — — 919.2 919.2 Other net revenues — — 2.4 2.4 Revenues before reimbursements 4,500.1 713.3 921.6 6,135.0 Reimbursements — 101.7 — 101.7 Total revenues $ 4,500.1 $ 815.0 $ 921.6 $ 6,236.7 Revenues by type and segment for the three-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 889.9 $ — $ — $ 889.9 Fees 293.2 202.6 — 495.8 Supplemental revenues 54.7 — — 54.7 Contingent revenues 34.5 — — 34.5 Investment income 18.9 0.1 — 19.0 Net gains on divestitures 3.4 — — 3.4 Revenues from clean coal activities — — 310.8 310.8 Other net losses — — (0.6 ) (0.6 ) Revenues before reimbursements 1,294.6 202.7 310.2 1,807.5 Reimbursements — 41.6 — 41.6 Total revenues $ 1,294.6 $ 244.3 $ 310.2 $ 1,849.1 Revenues by type and segment for the nine-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 2,734.6 $ — $ — $ 2,734.6 Fees 858.1 604.7 — 1,462.8 Supplemental revenues 164.0 — — 164.0 Contingent revenues 117.0 — — 117.0 Investment income 53.0 0.6 — 53.6 Net gains on divestitures 4.6 — — 4.6 Revenues from clean coal activities — — 652.1 652.1 Other net losses — — (0.4 ) (0.4 ) Revenues before reimbursements 3,931.3 605.3 651.7 5,188.3 Reimbursements — 111.7 — 111.7 Total revenues $ 3,931.3 $ 717.0 $ 651.7 $ 5,300.0
Summary of Geographical Location and Segment	Revenues by geographical location and segment for the three-month period ended September 30, 2021 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 1,000.3 $ 233.9 $ 357.9 $ 1,592.1 United Kingdom 276.5 12.1 — 288.6 Australia 62.4 29.6 — 92.0 Canada 68.1 1.6 — 69.7 New Zealand 41.0 3.4 — 44.4 Other foreign 51.4 — — 51.4 Total revenues $ 1,499.7 $ 280.6 $ 357.9 $ 2,138.2 Revenues by geographical location and segment for the nine-month period ended September 30, 2021 Risk Brokerage Management Corporate Total Revenues: United States $ 2,919.3 $ 672.9 $ 921.6 $ 4,513.8 United Kingdom 895.0 35.0 — 930.0 Australia 187.1 91.6 — 278.7 Canada 215.5 4.4 — 219.9 New Zealand 118.9 11.1 — 130.0 Other foreign 164.3 — — 164.3 Total revenues $ 4,500.1 $ 815.0 $ 921.6 $ 6,236.7 Revenues by type and segment for the nine-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: Commissions $ 2,734.6 $ — $ — $ 2,734.6 Fees 858.1 604.7 — 1,462.8 Supplemental revenues 164.0 — — 164.0 Contingent revenues 117.0 — — 117.0 Investment income 53.0 0.6 — 53.6 Net gains on divestitures 4.6 — — 4.6 Revenues from clean coal activities — — 652.1 652.1 Other net losses — — (0.4 ) (0.4 ) Revenues before reimbursements 3,931.3 605.3 651.7 5,188.3 Reimbursements — 111.7 — 111.7 Total revenues $ 3,931.3 $ 717.0 $ 651.7 $ 5,300.0 Revenues by geographical location and segment for the three-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 866.6 $ 203.9 $ 310.2 $ 1,380.7 United Kingdom 230.8 9.8 — 240.6 Australia 56.6 26.2 — 82.8 Canada 57.1 1.4 — 58.5 New Zealand 37.0 3.0 — 40.0 Other foreign 46.5 — — 46.5 Total revenues $ 1,294.6 $ 244.3 $ 310.2 $ 1,849.1 Revenues by geographical location and segment for the nine-month period ended September 30, 2020 are as follows (in millions): Risk Brokerage Management Corporate Total Revenues: United States $ 2,613.3 $ 602.2 $ 651.7 $ 3,867.2 United Kingdom 746.2 29.4 — 775.6 Australia 156.6 72.5 — 229.1 Canada 171.5 3.9 — 175.4 New Zealand 103.0 9.0 — 112.0 Other foreign 140.7 — — 140.7 Total revenues $ 3,931.3 $ 717.0 $ 651.7 $ 5,300.0

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2021CountrySegmentFacility
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of reportable segments | Segment	3
Number of commercial clean coal production facilities | Facility	35
Number of countries in which the company has operations	57
Number of countries in which the company does business through a network of correspondent brokers and consultants	150

Effect of New Accounting Pron_2

Effect of New Accounting Pronouncements - Additional Information (Details) - ASU 2019-12 [Member]	Sep. 30, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 1, 2021
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true

Business Combinations - Acquisi

Business Combinations - Acquisition Method for Recording Business Combinations (Detail)	9 Months Ended
	Sep. 30, 2021USD ($)shares
Atlas General Holdings LLC [Member]
Business Acquisition [Line Items]
Cash Paid	$ 95,200,000
Escrow Deposited	5,400,000
Recorded Earnout Payable	8,700,000
Total Recorded Purchase Price	109,300,000
Maximum Potential Earnout Payable	35,000,000
Bollington Wilson Group [Member]
Business Acquisition [Line Items]
Cash Paid	328,000,000
Escrow Deposited	1,400,000
Total Recorded Purchase Price	329,400,000
LDJ American Online Benefits Group, LLC [Member]
Business Acquisition [Line Items]
Cash Paid	38,200,000
Escrow Deposited	7,100,000
Recorded Earnout Payable	14,800,000
Total Recorded Purchase Price	60,100,000
Maximum Potential Earnout Payable	$ 20,000,000
Sixteen Other Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	337,000
Common Share Value	$ 44,400,000
Cash Paid	157,300,000
Accrued Liability	13,100,000
Escrow Deposited	18,900,000
Recorded Earnout Payable	34,100,000
Total Recorded Purchase Price	267,800,000
Maximum Potential Earnout Payable	$ 70,200,000
2021 Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	337,000
Common Share Value	$ 44,400,000
Cash Paid	618,700,000
Accrued Liability	13,100,000
Escrow Deposited	32,800,000
Recorded Earnout Payable	57,600,000
Total Recorded Purchase Price	766,600,000
Maximum Potential Earnout Payable	$ 125,200,000

Business Combinations - Additio

Business Combinations - Additional Information (Detail) shares in Millions	Aug. 13, 2021USD ($)	Jul. 29, 2021	May 20, 2021USD ($)	May 17, 2021USD ($)shares	Sep. 30, 2021USD ($)Entity	Jun. 30, 2021shares	Sep. 30, 2020USD ($)Entity	Sep. 30, 2021USD ($)Entity	Sep. 30, 2020USD ($)Entity	Sep. 30, 2021USD ($)Entity	Dec. 31, 2020USD ($)
Business Acquisition [Line Items]
Net cash raised through follow-on common stock offering								$ 1,524,600,000	$ 83,600,000
Aggregate principal amount					$ 5,532,100,000			5,532,100,000		$ 5,532,100,000	$ 4,551,600,000
Loss on extinguishment of debt	$ 16,200,000				16,200,000			16,200,000
Accretion of the discount on acquisition					8,200,000		$ 5,900,000	26,400,000	24,300,000
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations					$ (26,000,000)		$ (14,200,000)	$ (36,600,000)	$ 77,400,000
Number of companies acquired | Entity					44		55	80	126	602
Aggregate amount of maximum earnout obligations related to acquisitions								$ 1,026,100,000	$ 1,026,100,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet								578,700,000	578,700,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option					$ 560,500,000			560,500,000		$ 560,500,000
Aggregate amount of earnout obligation expected settlement in cash					18,200,000			18,200,000		18,200,000
Goodwill					453,500,000			453,500,000		453,500,000
Expiration lists					279,900,000			279,900,000		279,900,000
Non-compete agreements					3,800,000			3,800,000		3,800,000
Trade names					4,700,000			4,700,000		4,700,000
Total revenues					2,140,700,000		$ 1,899,600,000	6,278,400,000	5,435,700,000
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Goodwill					453,500,000			453,500,000		453,500,000
Expiration lists					279,900,000			279,900,000		279,900,000
Non-compete agreements					3,800,000			3,800,000		3,800,000
Trade names					4,700,000			4,700,000		4,700,000
Impairment wrote off of amortizable assets					0
Brokerage [Member]
Business Acquisition [Line Items]
Impairment wrote off of amortizable assets							$ 3,700,000	13,100,000	$ 49,700,000
Brokerage [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					118,000,000			118,000,000		118,000,000
Brokerage [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					1,600,000			1,600,000		1,600,000
Brokerage [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					$ 3,500,000			$ 3,500,000		$ 3,500,000
Minimum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								2 years
Minimum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								2 years
Minimum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								2 years
Maximum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								15 years
Maximum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								6 years
Maximum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years								15 years
Common Stock [Member]
Business Acquisition [Line Items]
Stock issuance from public offering (in shares) | shares						10.3
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]
Business Acquisition [Line Items]
Periodic payment of interest					3.50%			3.50%		3.50%
Unsecured Senior Notes [Member]
Business Acquisition [Line Items]
Aggregate principal amount			$ 1,500,000,000		$ 850,000,000			$ 850,000,000		$ 850,000,000
Weighted average interest rate			3.31%
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member]
Business Acquisition [Line Items]
Aggregate principal amount			$ 650,000,000
Periodic payment of interest			2.50%
Periodic payments of interest and principal, expiry year			2031
Notes redemption date		Aug. 13, 2021
Loss on extinguishment of debt	16,200,000
Unsecured Senior Notes [Member] | Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]
Business Acquisition [Line Items]
Aggregate principal amount			$ 850,000,000		$ 850,000,000			$ 850,000,000		$ 850,000,000
Periodic payment of interest			3.50%
Periodic payments of interest and principal, expiry year			2051
Willis Towers Watson PLC [Member]
Business Acquisition [Line Items]
Initial gross consideration	3,250,000,000
Potential additional consideration subject to certain revenue targets	750,000,000
Net cash raised through follow-on common stock offering	1,400,000,000
Cash borrowed through debt issuance and additional cash generated	$ 850,000,000
Debt instrument term	30 years
Agreement termination date		Jul. 29, 2021
Willis Towers Watson PLC [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Stock issuance from public offering (in shares) | shares				10.3
Net proceeds after underwriting, discounts and other expense				$ 1,437,900,000
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					2.5			2.5		2.5
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					15			15		15
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					3			3		3
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					4.8			4.8		4.8
2021 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]
Business Acquisition [Line Items]
Attrition rate								4.30%
2021 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]
Business Acquisition [Line Items]
Attrition rate								16.10%
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					7			7		7
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					10.5			10.5		10.5
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					9			9		9
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					17			17		17
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					3			3		3
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					4.8			4.8		4.8
2020 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]
Business Acquisition [Line Items]
Attrition rate								4.30%
2020 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]
Business Acquisition [Line Items]
Attrition rate								16.10%
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					9			9		9
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					17			17		17
Business Acquisition [Member]
Business Acquisition [Line Items]
Trade names					$ 4,700,000			$ 4,700,000		$ 4,700,000
Annualized revenue of business acquisitions								189,900,000
Total revenues								95,800,000
Net earnings								3,300,000
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists					279,900,000			279,900,000		279,900,000
Non-compete agreements					3,800,000			3,800,000		3,800,000
Deferred tax liability					$ 20,200,000			$ 20,200,000		$ 20,200,000

Business Combinations - Summary

Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Sep. 30, 2021USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	$ 69.4
Other current assets	194.6
Fixed assets	11.4
Noncurrent assets	12.9
Goodwill	453.5
Expiration lists	279.9
Non-compete agreements	3.8
Trade names	4.7
Total assets acquired	1,030.2
Current liabilities	227.3
Noncurrent liabilities	36.3
Total liabilities assumed	263.6
Total net assets acquired	766.6
Atlas General Holdings LLC [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	32.8
Other current assets	112
Fixed assets	7.4
Noncurrent assets	1.2
Goodwill	45.9
Expiration lists	39.7
Non-compete agreements	0.3
Total assets acquired	239.3
Current liabilities	129.8
Noncurrent liabilities	0.2
Total liabilities assumed	130
Total net assets acquired	109.3
Bollington Wilson Group [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	24.9
Other current assets	15.3
Fixed assets	3.8
Noncurrent assets	5.3
Goodwill	257.1
Expiration lists	81.6
Non-compete agreements	1.2
Trade names	3.4
Total assets acquired	392.6
Current liabilities	39.6
Noncurrent liabilities	23.6
Total liabilities assumed	63.2
Total net assets acquired	329.4
LDJ American Online Benefits Group, LLC [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	0.7
Other current assets	15.8
Noncurrent assets	0.2
Goodwill	25.7
Expiration lists	25.4
Non-compete agreements	0.2
Trade names	0.2
Total assets acquired	68.2
Current liabilities	8
Noncurrent liabilities	0.1
Total liabilities assumed	8.1
Total net assets acquired	60.1
Sixteen Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and restricted cash	11
Other current assets	51.5
Fixed assets	0.2
Noncurrent assets	6.2
Goodwill	124.8
Expiration lists	133.2
Non-compete agreements	2.1
Trade names	1.1
Total assets acquired	330.1
Current liabilities	49.9
Noncurrent liabilities	12.4
Total liabilities assumed	62.3
Total net assets acquired	$ 267.8

Business Combinations - Summa_2

Business Combinations - Summary of Unaudited Pro Forma Historical Results (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Combinations [Abstract]
Total revenues	$ 2,140.7	$ 1,899.6	$ 6,278.4	$ 5,435.7
Net earnings attributable to controlling interests	$ 225	$ 181.6	$ 801.9	$ 685.9
Basic net earnings per share	$ 1.09	$ 0.94	$ 3.99	$ 3.60
Diluted net earnings per share	$ 1.06	$ 0.92	$ 3.90	$ 3.52

Contracts with Customers - Summ

Contracts with Customers - Summary of Unbilled Receivables, Contract Assets and Contract Liabilities from Contracts with Customers (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Contract With Customer Asset And Liability [Abstract]
Unbilled receivables	$ 810.6	$ 603.1
Deferred contract costs	76.3	102
Deferred revenue	$ 588.2	$ 541.3

Contracts with Customers - Su_2

Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 541.3
Incremental deferred revenue	360.7
Revenue recognized during the nine-month period ended September 30, 2021 included in deferred revenue at December 31, 2020	(333.3)
Net change in collected billings/deposits received from customers	8.8
Impact of change in foreign exchange rates	1
Deferred revenue recognized from business acquisitions	9.7
Deferred revenue ending balance	588.2
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	354.7
Incremental deferred revenue	283.3
Revenue recognized during the nine-month period ended September 30, 2021 included in deferred revenue at December 31, 2020	(256.5)
Net change in collected billings/deposits received from customers	1.5
Impact of change in foreign exchange rates	1
Deferred revenue recognized from business acquisitions	9.7
Deferred revenue ending balance	393.7
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	186.6
Incremental deferred revenue	77.4
Revenue recognized during the nine-month period ended September 30, 2021 included in deferred revenue at December 31, 2020	(76.8)
Net change in collected billings/deposits received from customers	7.3
Deferred revenue ending balance	$ 194.5

Contracts with Customers - Addi

Contracts with Customers - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Revenue From Contract With Customer [Abstract]
Remaining performance obligations	$ 588.2
Deferred contract costs	76.3	$ 102
Amortization of deferred contract costs	$ 328.5		$ 304

Contracts with Customers - Su_3

Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail) $ in Millions	Sep. 30, 2021USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 588.2
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 325
Remaining performance obligation, expected timing of satisfaction, period	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 190.4
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 42.7
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 12.1
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 6.7
Remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 11.3
Remaining performance obligation, expected timing of satisfaction, period
Brokerage [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 393.7
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 235.6
Remaining performance obligation, expected timing of satisfaction, period	3 months
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 135.8
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 19.3
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 1.5
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 0.8
Remaining performance obligation, expected timing of satisfaction, period	1 year
Brokerage [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 0.7
Remaining performance obligation, expected timing of satisfaction, period
Risk Management [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 194.5
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 89.4
Remaining performance obligation, expected timing of satisfaction, period	3 months
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 54.6
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 23.4
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 10.6
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 5.9
Remaining performance obligation, expected timing of satisfaction, period	1 year
Risk Management [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 10.6
Remaining performance obligation, expected timing of satisfaction, period

Contracts with Customers - Su_4

Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations 1 (Detail) $ in Millions	Sep. 30, 2021USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 588.2
Brokerage [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	393.7
Risk Management [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 194.5

Other Financial Data - Summary

Other Financial Data - Summary of Major Classes of Other Current Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Premium finance advances and loans	$ 511.8	$ 442.7
Accrued supplemental, direct bill and other receivables	546.2	361.7
Refined coal production related receivables	108.5	95.4
Deferred contract costs	76.3	102
Prepaid expenses	126.7	112.1
Total other current assets	$ 1,369.5	$ 1,113.9

Intangible Assets - Carrying Am

Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Goodwill [Line Items]
Total goodwill	$ 6,627.5	$ 6,127
Unites States [Member]
Goodwill [Line Items]
Total goodwill	3,633.7	3,433
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	1,621.8	1,343.4
Canada [Member]
Goodwill [Line Items]
Total goodwill	523	492.9
Australia [Member]
Goodwill [Line Items]
Total goodwill	461	473.6
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	231.1	232.6
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	156.9	151.5
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	6,522.8	6,053.6
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	3,568.9	3,399.8
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	1,606.4	1,328.3
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill	523	492.9
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	450	462.1
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	220.5	221.9
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	154	148.6
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	101.8	70.5
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	64.8	33.2
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	15.4	15.1
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	11	11.5
Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	10.6	10.7
Corporate [Member]
Goodwill [Line Items]
Total goodwill	2.9	2.9
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 2.9	$ 2.9

Intangible Assets - Changes in

Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Goodwill [Line Items]
Beginning Balance	$ 6,127
Goodwill acquired during the period	453.5
Goodwill adjustments due to appraisals and other acquisition adjustments	50.5
Goodwill written-off related to sales of business and impairment	(1)
Foreign currency translation adjustments during the period	(2.5)
Ending Balance	6,627.5
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	6,053.6
Goodwill acquired during the period	421.9
Goodwill adjustments due to appraisals and other acquisition adjustments	50.5
Goodwill written-off related to sales of business and impairment	(1)
Foreign currency translation adjustments during the period	(2.2)
Ending Balance	6,522.8
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	70.5
Goodwill acquired during the period	31.6
Foreign currency translation adjustments during the period	(0.3)
Ending Balance	101.8
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.9
Ending Balance	$ 2.9

Intangible Assets - Major Class

Intangible Assets - Major Classes of Amortizable Intangible Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Amortizable intangible assets, net	$ 2,339	$ 2,399.9
Expiration Lists [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable intangible assets, gross	4,997.2	4,753.2
Accumulated amortization	(2,727.5)	(2,436.7)
Amortizable intangible assets, net	2,269.7	2,316.5
Non-Compete Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable intangible assets, gross	73.5	75.9
Accumulated amortization	(61.9)	(57.8)
Amortizable intangible assets, net	11.6	18.1
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable intangible assets, gross	105.5	107.8
Accumulated amortization	(47.8)	(42.5)
Amortizable intangible assets, net	$ 57.7	$ 65.3

Intangible Assets - Estimated A

Intangible Assets - Estimated Aggregate Amortization Expense (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
2021 (remaining three months)	$ 96
2022	367.7
2023	342.4
2024	306.1
2025	266.2
Thereafter	960.6
Amortizable intangible assets, net	$ 2,339	$ 2,399.9

Credit and Other Debt Agreeme_3

Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2021	May 20, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 5,532.1		$ 4,551.6
Less unamortized debt acquisition costs on Senior Notes and Note Purchase Agreements	(15.6)		(7)
Less unamortized discount on Bonds Payable	(10)
Semi-annual payments of interest, Net	5,506.5		4,544.6
Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	4,448		4,348
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	850	$ 1,500
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	850	$ 850
Fixed Rate of 5.18%, Balloon Due February 10, 2021 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest			75
Fixed Rate of 3.69%, Balloon Due June 14, 2022 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200		200
Fixed Rate of 5.49%, Balloon Due February 10, 2023 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50		50
Fixed Rate of 4.13%, Balloon Due June 24, 2023 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200		200
Fixed Rate of 4.72%, Balloon due February 13, 2024 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100		100
Fixed Rate of 4.58%, Balloon Due February 27, 2024 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	325		325
Floating Interest Rate of 1.40% LIBOR Plus Balloon Due June 13, 2024 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50		50
Fixed Rate of 4.31%, Balloon Due June 24, 2025 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200		200
Fixed Rate of 4.85%, Balloon due February 13, 2026 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	140		140
Fixed Rate of 4.73%, Balloon Due February 27, 2026 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175		175
Fixed Rate of 4.40%, Balloon Due June 2, 2026 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175		175
Fixed Rate of 4.36%, Balloon Due June 24, 2026 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	150		150
Fixed Rate of 3.75%, Balloon Due January 30, 2027 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	30		30
Fixed Rate of 4.09%, Balloon Due June 27, 2027 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125		125
Fixed Rate of 4.09%, Balloon Due August 2, 2027 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125		125
Fixed Rate of 4.14%, Balloon Due August 4, 2027 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	98		98
Fixed Rate of 3.46%, Balloon Due December 1, 2027 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100		100
Fixed Rate of 4.55%, Balloon Due June 2, 2028 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75		75
Fixed Rate of 4.34%, Balloon due June 13, 2028 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125		125
Fixed Rate of 5.04%, Balloon due February 13, 2029 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100		100
Fixed Rate of 4.98%, Balloon Due February 27, 2029 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100		100
Fixed Rate of 4.19%, Balloon Due June 27, 2029 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50		50
Fixed Rate of 4.19%, Balloon Due August 2, 2029 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50		50
Fixed Rate of 3.48%, Balloon Due December 2, 2029 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50		50
Fixed Rate of 3.99%, Balloon Due January 30, 2030 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	341		341
Fixed Rate of 4.44%, Balloon Due June 13, 2030 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125		125
Fixed Rate of 5.14%, Balloon due March 13, 2031 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	180		180
Fixed Rate of 4.70%, Balloon Due June 2, 2031 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	25		25
Fixed Rate of 4.09%, Balloon Due January 30, 2032 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	69		69
Fixed Rate of 4.34%, Balloon Due June 27, 2032 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75		75
Fixed Rate of 4.34%, Balloon Due August 2, 2032 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75		75
Fixed Rate of 4.59%, Balloon due June 13, 2033 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125		125
Fixed Rate of 5.29%, balloon due March 13, 2034 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	40		40
Fixed Rate of 4.48%, Balloon Due June 12, 2034 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175		175
Fixed Rate of 4.24%, Balloon Due January 30, 2035 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	79		79
Fixed Rate of 2.44%, Balloon Due February 10, 2036 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100
Fixed Rate of 2.46%, Balloon Due May 5, 2036 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75
Fixed Rate of 4.69%, Balloon Due June 13, 2038 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75		75
Fixed Rate of 5.45%, balloon due March 13, 2039 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	40		40
Interbank Rates Plus 1.200% Expires September 15, 2023 [Member] | AUD Denominated Tranche [Member] | Facility B [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	211.9		193.1
Fixed Rate of 4.49%, Balloon Due January 30, 2040 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	56		56
Interbank Rates Plus 0.680% Expires September 15, 2023 [Member] | AUD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	11.8
Interbank Rates Plus 0.940% Expires September 15, 2023 [Member] | NZD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	10.4		10.5
Interbank Rates Expires September 15, 2023 [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 234.1		$ 203.6

Credit and Other Debt Agreeme_4

Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Periodic payments of interest and principal, expiry date	Sep. 15, 2023	Sep. 15, 2023
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.50%
Periodic payments of interest and principal, expiry date	May 20, 2051
Fixed Rate of 5.18%, Balloon Due February 10, 2021 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.18%	5.18%
Periodic payments of interest and principal, expiry date	Feb. 10, 2021	Feb. 10, 2021
Fixed Rate of 3.69%, Balloon Due June 14, 2022 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.69%	3.69%
Periodic payments of interest and principal, expiry date	Jun. 14, 2022	Jun. 14, 2022
Fixed Rate of 5.49%, Balloon Due February 10, 2023 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.49%	5.49%
Periodic payments of interest and principal, expiry date	Feb. 10, 2023	Feb. 10, 2023
Fixed Rate of 4.13%, Balloon Due June 24, 2023 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.13%	4.13%
Periodic payments of interest and principal, expiry date	Jun. 24, 2023	Jun. 24, 2023
Fixed Rate of 4.72%, Balloon due February 13, 2024 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.72%	4.72%
Periodic payments of interest and principal, expiry date	Feb. 13, 2024	Feb. 13, 2024
Fixed Rate of 4.58%, Balloon Due February 27, 2024 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.58%	4.58%
Periodic payments of interest and principal, expiry date	Feb. 27, 2024	Feb. 27, 2024
Floating Interest Rate of 1.40% LIBOR Plus Balloon Due June 13, 2024 [Member]
Debt Instrument [Line Items]
Periodic payments of interest and principal, expiry date	Jun. 13, 2024	Jun. 13, 2024
Quarterly payments of interest rate	1.40%	1.40%
Quarterly payments of interest, description	90 day LIBOR plus
Fixed Rate of 4.31%, Balloon Due June 24, 2025 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.31%	4.31%
Periodic payments of interest and principal, expiry date	Jun. 24, 2025	Jun. 24, 2025
Fixed Rate of 4.85%, Balloon due February 13, 2026 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.85%	4.85%
Periodic payments of interest and principal, expiry date	Feb. 13, 2026	Feb. 13, 2026
Fixed Rate of 4.73%, Balloon Due February 27, 2026 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.73%	4.73%
Periodic payments of interest and principal, expiry date	Feb. 27, 2026	Feb. 27, 2026
Fixed Rate of 4.40%, Balloon Due June 2, 2026 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.40%	4.40%
Periodic payments of interest and principal, expiry date	Jun. 2, 2026	Jun. 2, 2026
Fixed Rate of 4.36%, Balloon Due June 24, 2026 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.36%	4.36%
Periodic payments of interest and principal, expiry date	Jun. 24, 2026	Jun. 24, 2026
Fixed Rate of 3.75%, Balloon Due January 30, 2027 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.75%	3.75%
Periodic payments of interest and principal, expiry date	Jan. 30, 2027	Jan. 30, 2027
Fixed Rate of 4.09%, Balloon Due June 27, 2027 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.09%	4.09%
Periodic payments of interest and principal, expiry date	Jun. 27, 2027	Jun. 27, 2027
Fixed Rate of 4.09%, Balloon Due August 2, 2027 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.09%	4.09%
Periodic payments of interest and principal, expiry date	Aug. 2, 2027	Aug. 2, 2027
Fixed Rate of 4.14%, Balloon Due August 4, 2027 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.14%	4.14%
Periodic payments of interest and principal, expiry date	Aug. 4, 2027	Aug. 4, 2027
Fixed Rate of 3.46%, Balloon Due December 1, 2027 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.46%	3.46%
Periodic payments of interest and principal, expiry date	Dec. 1, 2027	Dec. 1, 2027
Fixed Rate of 4.55%, Balloon Due June 2, 2028 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.55%	4.55%
Periodic payments of interest and principal, expiry date	Jun. 2, 2028	Jun. 2, 2028
Fixed Rate of 4.34%, Balloon due June 13, 2028 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.34%	4.34%
Periodic payments of interest and principal, expiry date	Jun. 13, 2028	Jun. 13, 2028
Fixed Rate of 5.04%, Balloon due February 13, 2029 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.04%	5.04%
Periodic payments of interest and principal, expiry date	Feb. 13, 2029	Feb. 13, 2029
Fixed Rate of 4.98%, Balloon Due February 27, 2029 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.98%	4.98%
Periodic payments of interest and principal, expiry date	Feb. 27, 2029	Feb. 27, 2029
Fixed Rate of 4.19%, Balloon Due June 27, 2029 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.19%	4.19%
Periodic payments of interest and principal, expiry date	Jun. 27, 2029	Jun. 27, 2029
Fixed Rate of 4.19%, Balloon Due August 2, 2029 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.19%	4.19%
Periodic payments of interest and principal, expiry date	Aug. 2, 2029	Aug. 2, 2029
Fixed Rate of 3.48%, Balloon Due December 2, 2029 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.48%	3.48%
Periodic payments of interest and principal, expiry date	Dec. 2, 2029	Dec. 2, 2029
Fixed Rate of 3.99%, Balloon Due January 30, 2030 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	3.99%	3.99%
Periodic payments of interest and principal, expiry date	Jan. 30, 2030	Jan. 30, 2030
Fixed Rate of 4.44%, Balloon Due June 13, 2030 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.44%	4.44%
Periodic payments of interest and principal, expiry date	Jun. 13, 2030	Jun. 13, 2030
Fixed Rate of 5.14%, Balloon due March 13, 2031 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.14%	5.14%
Periodic payments of interest and principal, expiry date	Mar. 13, 2031	Mar. 13, 2031
Fixed Rate of 4.70%, Balloon Due June 2, 2031 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.70%	4.70%
Periodic payments of interest and principal, expiry date	Jun. 2, 2031	Jun. 2, 2031
Fixed Rate of 2.46%, Balloon Due May 5, 2036 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	2.46%	2.46%
Periodic payments of interest and principal, expiry date	May 5, 2036	May 5, 2036
Fixed Rate of 4.09%, Balloon Due January 30, 2032 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.09%	4.09%
Periodic payments of interest and principal, expiry date	Jan. 30, 2032	Jan. 30, 2032
Fixed Rate of 4.34%, Balloon Due June 27, 2032 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.34%	4.34%
Periodic payments of interest and principal, expiry date	Jun. 27, 2032	Jun. 27, 2032
Fixed Rate of 4.34%, Balloon Due August 2, 2032 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.34%	4.34%
Periodic payments of interest and principal, expiry date	Aug. 2, 2032	Aug. 2, 2032
Fixed Rate of 4.59%, Balloon due June 13, 2033 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.59%	4.59%
Periodic payments of interest and principal, expiry date	Jun. 13, 2033	Jun. 13, 2033
Fixed Rate of 5.29%, balloon due March 13, 2034 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.29%	5.29%
Periodic payments of interest and principal, expiry date	Mar. 13, 2034	Mar. 13, 2034
Fixed Rate of 4.48%, Balloon Due June 12, 2034 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.48%	4.48%
Periodic payments of interest and principal, expiry date	Jun. 12, 2034	Jun. 12, 2034
Fixed Rate of 4.24%, Balloon Due January 30, 2035 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.24%	4.24%
Periodic payments of interest and principal, expiry date	Jan. 30, 2035	Jan. 30, 2035
Fixed Rate of 2.44%, Balloon Due February 10, 2036 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	2.44%	2.44%
Periodic payments of interest and principal, expiry date	Feb. 10, 2036	Feb. 10, 2036
Fixed Rate of 4.69%, Balloon Due June 13, 2038 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.69%	4.69%
Periodic payments of interest and principal, expiry date	Jun. 13, 2038	Jun. 13, 2038
Fixed Rate of 5.45%, balloon due March 13, 2039 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	5.45%	5.45%
Periodic payments of interest and principal, expiry date	Mar. 13, 2039	Mar. 13, 2039
Fixed Rate of 4.49%, Balloon Due January 30, 2040 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	4.49%	4.49%
Periodic payments of interest and principal, expiry date	Jan. 30, 2040	Jan. 30, 2040
Prime or LIBOR Plus up to 1.45%, Expires June 7, 2024 [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	1.45%	1.45%
Periodic payments of interest and principal, expiry date	Jun. 7, 2024	Jun. 7, 2024
Facility B [Member] | Interbank Rates Plus 1.200% Expires September 15, 2023 [Member] | Premium Financing Debt Facility [Member] | AUD Denominated Tranche [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	1.20%	1.20%
Facility B [Member] | Interbank Rates Plus 1.650% Expires September 15, 2023 [Member] | Premium Financing Debt Facility [Member] | NZD Denominated Tranche [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	1.65%	1.65%
Facility C and D [Member] | Interbank Rates Plus 0.680% Expires September 15, 2023 [Member] | Premium Financing Debt Facility [Member] | AUD Denominated Tranche [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	0.68%	0.68%
Facility C and D [Member] | Interbank Rates Plus 0.940% Expires September 15, 2023 [Member] | Premium Financing Debt Facility [Member] | NZD Denominated Tranche [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest	0.94%	0.94%

Credit and Other Debt Agreeme_5

Credit and Other Debt Agreements - Additional Information (Detail) - USD ($) $ in Millions	Aug. 13, 2021	Jul. 29, 2021	May 20, 2021	Sep. 30, 2021	Sep. 30, 2021
Debt Instrument [Line Items]
Loss on extinguishment of debt	$ 16.2			$ 16.2	$ 16.2
Willis Towers Watson PLC [Member]
Debt Instrument [Line Items]
Agreement termination date		Jul. 29, 2021
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 1,500
Weighted average interest rate			3.31%
Realized a cash gain (loss) on hedging transaction			$ (57.8)
Derivative instrument, term			10 years
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 650
Periodic Payment of Interest			2.50%
Periodic payments of interest and principal, expiry year			2031
Amount payable to redeem the notes, percent of the principal amount following transaction termination event			101.00%
Number of days to occur reinsurance transaction termination event for note redemption			30 days
Notes redemption date		Aug. 13, 2021
Loss on extinguishment of debt	16.2
Redemption premium	6.5
Unamortized discount amount debt issuance and write-off debt acquisition costs	$ 9.7
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest				3.50%	3.50%
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 850
Periodic Payment of Interest			3.50%
Periodic payments of interest and principal, expiry year			2051

Earnings Per Share - Computatio

Earnings Per Share - Computation of Basic and Diluted Net EPS (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net earnings attributable to controlling interests	$ 225.1	$ 176.6	$ 797.4	$ 676.6
Weighted average number of common shares outstanding	207	191.9	201	190.4
Dilutive effect of stock options using the treasury stock method	4.5	4.4	4.5	4.1
Weighted average number of common and common equivalent shares outstanding	211.5	196.3	205.5	194.5
Basic net earnings per share	$ 1.09	$ 0.92	$ 3.97	$ 3.55
Diluted net earnings per share	$ 1.06	$ 0.90	$ 3.88	$ 3.48

Earnings Per Share - Additional

Earnings Per Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Anti-dilutive stock-based awards shares outstanding	1.6	0	1.2	1

Stock Option Plans - Additional

Stock Option Plans - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions	Mar. 16, 2021	Mar. 12, 2020	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Stock Option Plans [Line Items]
Compensation expense related to stock option grants			$ 4.7	$ 4.5	$ 3.7	$ 3.4	$ 3.3	$ 3.4	$ 12.9	$ 10.1
Total intrinsic value of options exercised									93.6	$ 68.7
Total unrecognized compensation cost related to nonvested options			$ 52.4						$ 52.4
Weighted average period, years									4 years
Black-Scholes Option Pricing Model [Member]
Stock Option Plans [Line Items]
Weighted average fair value per option for all options									$ 23.38	$ 9.99
Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Maximum number of shares available									1,800,000
Shares available for grant			9,500,000						9,500,000
Long Term Incentive Plan [Member] | Officer and Key Employees [Member]
Stock Option Plans [Line Items]
Shares available for grant	1,640,000	1,590,740
Stock options granted, exercise percentage, on the third anniversary date of the grant	34.00%	34.00%
Stock options granted, exercise percentage, on the fourth anniversary date of the grant	33.00%	33.00%
Stock options granted, exercise percentage, on the fifth anniversary date of the grant	33.00%	33.00%
Long Term Incentive Plan [Member] | Maximum [Member]
Stock Option Plans [Line Items]
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant									100.00%
Maximum period for the exercise of stock options, years									7 years	7 years
Long Term Incentive Plan [Member] | Minimum [Member]
Stock Option Plans [Line Items]
Period of service from grant date stock options awarded not subject to forfeiture									2 years	2 years
Long Term Incentive Plan [Member] | Minimum [Member] | Executive Officer [Member]
Stock Option Plans [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance									55 years	55 years
2017 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Number Of Periods Options Expire									7 years

Stock Option Plans - Black-Scho

Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected dividend yield	$ 1.5	$ 2.1
Expected risk-free interest rate	0.90%	0.70%
Volatility	22.90%	17.30%
Expected life (in years)	5 years 4 months 24 days	5 years 4 months 24 days

Stock Option Plans - Stock Opti

Stock Option Plans - Stock Option Activity and Related Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Under Option, Beginning balance	7.5
Shares Under Option, Granted	1.6
Shares Under Option, Exercised	(1.1)
Shares Under Option, Forfeited or canceled	(0.1)
Shares Under Option, Ending balance	7.9
Shares Under Option, Exercisable at end of year	2.3
Shares Under Option, Ending unvested and expected to vest	5.1
Weighted Average Exercise Price, Beginning balance	$ 65.09
Weighted Average Exercise Price, Granted	127.90
Weighted Average Exercise Price, Exercised	49.30
Weighted Average Exercise Price, Forfeited or canceled	86.21
Weighted Average Exercise Price, Ending balance	80.17
Weighted Average Exercise Price, Exercisable at end of year	51.05
Weighted Average Exercise Price, Ending unvested and expected to vest	$ 91.53
Weighted Average Remaining Contractual Term (in years), Ending balance	4 years 18 days
Weighted Average Remaining Contractual Term (in years), Exercisable at end of year	1 year 9 months 3 days
Weighted Average Remaining Contractual Term (in years), Ending unvested and expected to vest	4 years 11 months 19 days
Aggregate Intrinsic Value, Ending Balance	$ 539.1
Aggregate Intrinsic Value, Exercisable at end of year	228
Aggregate Intrinsic Value, Ending unvested and expected to vest	$ 293.8

Stock Option Plans - Stock Op_2

Stock Option Plans - Stock Options Outstanding and Exercisable (Detail) - $ / shares shares in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	$ 43.71
Range of Exercise Prices, maximum	$ 127.90
Option Outstanding, Number Outstanding	7.9
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	4 years 18 days
Option Outstanding, Weighted Average Exercise Price	$ 80.17	$ 65.09
Options Exercisable, Number Exercisable	2.3
Option Exercisable, Weighted Average Exercise Price	$ 51.05
Exercise Prices Range $ 43.71 - $ 43.71 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	43.71
Range of Exercise Prices, maximum	$ 43.71
Option Outstanding, Number Outstanding	1
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	1 year 5 months 15 days
Option Outstanding, Weighted Average Exercise Price	$ 43.71
Options Exercisable, Number Exercisable	0.9
Option Exercisable, Weighted Average Exercise Price	$ 43.71
Exercise Prices Range $ 46.17 - $ 46.17 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	46.17
Range of Exercise Prices, maximum	$ 46.17
Option Outstanding, Number Outstanding	0.5
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	5 months 8 days
Option Outstanding, Weighted Average Exercise Price	$ 46.17
Options Exercisable, Number Exercisable	0.5
Option Exercisable, Weighted Average Exercise Price	$ 46.17
Exercise Prices Range $ 49.55 - $ 56.86 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	49.55
Range of Exercise Prices, maximum	$ 56.86
Option Outstanding, Number Outstanding	1.1
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	2 years 5 months 15 days
Option Outstanding, Weighted Average Exercise Price	$ 56.83
Options Exercisable, Number Exercisable	0.6
Option Exercisable, Weighted Average Exercise Price	$ 56.81
Exercise Prices Range $ 70.74 - $ 70.74 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	70.74
Range of Exercise Prices, maximum	$ 70.74
Option Outstanding, Number Outstanding	1
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	3 years 5 months 15 days
Option Outstanding, Weighted Average Exercise Price	$ 70.74
Options Exercisable, Number Exercisable	0.3
Option Exercisable, Weighted Average Exercise Price	$ 70.74
Exercise Prices Range $ 79.59 - $ 79.59 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	79.59
Range of Exercise Prices, maximum	$ 79.59
Option Outstanding, Number Outstanding	1.2
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	4 years 5 months 12 days
Option Outstanding, Weighted Average Exercise Price	$ 79.59
Option Exercisable, Weighted Average Exercise Price	79.59
Exercise Prices Range $ 86.17 - $ 86.17 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	86.17
Range of Exercise Prices, maximum	$ 86.17
Option Outstanding, Number Outstanding	1.5
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	5 years 5 months 12 days
Option Outstanding, Weighted Average Exercise Price	$ 86.17
Option Exercisable, Weighted Average Exercise Price	86.17
Exercise Prices Range $ 127.90 - $ 127.90 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, minimum	127.90
Range of Exercise Prices, maximum	$ 127.90
Option Outstanding, Number Outstanding	1.6
Option Outstanding, Weighted Average Remaining Contractual Term (in years)	6 years 5 months 15 days
Option Outstanding, Weighted Average Exercise Price	$ 127.90

Deferred Compensation - Additio

Deferred Compensation - Additional Information (Detail) - USD ($) shares in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Investments And Employee Deferred Compensation Plan [Line Items]
Deferred Equity Participation Plan, distributions to key executives, age					age 62 (or the one-year anniversary of the date of the grant for participants over the age of 61 as of the grant date) or upon or after their actual retirement if later.
Deferred Equity Participation Sub-plans, distributions requisite service description					we annually make awards under sub-plans of the DEPP for certain production staff, which generally provide for vesting and/or distributions no sooner than five years from the date of awards, although certain awards vest and/or distribute after the earlier of fifteen years or the participant reaching age 65.
Deferred Equity Participation Sub-plans, distributions to key executives, age					age 65
Deferred Equity Participation Plan (DEPP) [Member]
Investments And Employee Deferred Compensation Plan [Line Items]
Awards approved by committee, value		$ 17,000,000		$ 14,100,000
Charge to compensation expenses related to awards	$ 3,800,000		$ 3,000,000		$ 10,300,000	$ 9,000,000
Unearned deferred compensation, value					$ 74,300,000		$ 60,500,000
Unearned deferred compensation, shares					2.7		2.6
Total intrinsic value of unvested equity based awards					$ 402,800,000		$ 327,800,000
Cash and equity awards with aggregate fair value vested and distributed to participants					10,100,000	9,800,000
Deferred Equity Participation Plan Sub Plans [Member]
Investments And Employee Deferred Compensation Plan [Line Items]
Awards approved by committee, value		3,200,000		1,800,000
Charge to compensation expenses related to awards	500,000		700,000		1,900,000	2,100,000
Distributions from the sub-plans					0	0
Deferred Cash Participation Plan (DCPP) [Member]
Investments And Employee Deferred Compensation Plan [Line Items]
Awards approved by committee, value		$ 7,200,000		$ 3,000,000
Charge to compensation expenses related to awards	$ 2,900,000		$ 1,600,000		7,000,000	5,000,000
Cash and equity awards with aggregate fair value vested and distributed to participants					$ 6,700,000	$ 7,300,000

Restricted Stock, Performance_2

Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)

Mar. 16, 2021USD ($)shares

Mar. 12, 2020USD ($)shares

Sep. 30, 2021USD ($)shares

Mar. 31, 2021USD ($)shares

Sep. 30, 2020USD ($)shares

Mar. 31, 2020USD ($)shares

Mar. 31, 2019shares

Sep. 30, 2021USD ($)TIMEshares

Sep. 30, 2020USD ($)shares

Dec. 31, 2020shares

Mar. 14, 2019USD ($)shares

Mar. 15, 2018USD ($)shares

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Share-based compensation, shares outstanding | shares

7,900,000

7,900,000

7,500,000

Restricted Stock Units (RSUs) [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Shares granted in the period | shares

314,000

405,900

Fair value of grants in period

$ 40,100,000

$ 34,900,000

Share based payment award vesting date

Mar. 16, 2026

Mar. 12, 2025

Period of service from grant date stock options awarded not subject to forfeiture

2 years

2 years

Restricted stock or unit expense

$ 6,700,000

$ 5,500,000

$ 20,200,000

$ 17,000,000

Unvested Restricted Stock [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Total intrinsic value

293,400,000

221,100,000

293,400,000

221,100,000

Equity awards with an aggregate fair value

500,000

48,600,000

30,600,000

Performance Shares [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Restricted stock or unit expense

3,100,000

3,100,000

9,300,000

11,600,000

Total intrinsic value

48,300,000

35,400,000

48,300,000

35,400,000

Equity awards with an aggregate fair value

$ 19,100,000

12,500,000

Shares authorized | shares

67,000

82,500

Performance share awards approved, Fair value

$ 8,600,000

$ 7,100,000

Vesting period, years

2 years

2017 Performance Share Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Provisional awards, terms

The 2021 and 2020 awards are subject to a three-year performance period that began on January 1, 2021 and 2020, respectively, and vest on the three-year anniversary of the date of grant (March 16, 2024 and March 12, 2023).

2019 Provisional Cash Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Cash-based compensation awards, expenses

2,800,000

2,900,000

$ 8,800,000

7,200,000

Performance awards expiration date

Jan. 1, 2022

2020 Provisional Cash Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Cash-based compensation awards, expenses

$ 3,200,000

$ 9,400,000

Performance awards expiration date

Jan. 1, 2023

2018 Provisional Cash Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Provisional compensation cash award approved for future grant by compensation committee, units | shares

176,300

176,300

Cash-based compensation awards, expenses

$ 2,700,000

$ 17,700,000

$ 6,800,000

Performance awards expiration date

Jan. 1, 2021

2017 Provisional Cash Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Provisional compensation cash award approved for future grant by compensation committee, units | shares

221,600

221,600

Cash-based compensation awards, expenses

$ 18,900,000

Executive Officer [Member] | Restricted Stock Units (RSUs) [Member] | Minimum [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Minimum age of employee with not subject to award forfeiture on condition compliance

55 years

55 years

Officer and Key Employees [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Performance awards period, years

1 year

Officer and Key Employees [Member] | Cash Awards [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Provisional awards, terms

The ultimate award value will be equal to the trailing twelve-month price of our common stock on December 31, 2023, multiplied by the number of units subject to the award, but limited to between 0.5 and 1.5 times the original value of the units determined as of the grant date.

Vesting period, years

3 years

Provisional compensation cash awards approved for future grant by compensation committee, value

$ 18,800,000

$ 18,400,000

$ 16,500,000

$ 15,000,000

Provisional compensation cash award approved for future grant by compensation committee, units | shares

147,000

213,000

208,000

200,000

190,000

206,800

219,000

Ultimate award value, multiples of original value of the units, minimum | TIME

0.5

Ultimate award value, multiples of original value of the units, maximum | TIME

1.5

Cash-based compensation awards, expenses

$ 0

Long Term Incentive Plan [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Share-based compensation, shares outstanding, Value

$ 4,000,000

$ 4,000,000

Share-based compensation, shares outstanding | shares

1,800,000

1,800,000

Shares granted in the period | shares

1,800,000

Long Term Incentive Plan [Member] | Minimum [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Period of service from grant date stock options awarded not subject to forfeiture

2 years

2 years

Long Term Incentive Plan [Member] | Executive Officer [Member] | Minimum [Member]

Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]

Minimum age of employee with not subject to award forfeiture on condition compliance

55 years

55 years

Investments - Investments Repor

Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Assets	$ 14.1	$ 22.4
Funding Commitments	0.5
Chem-Mod LLC [Member]
Schedule of Equity Method Investments [Line Items]
Assets	4	4
Chem-Mod International LLC [Member]
Schedule of Equity Method Investments [Line Items]
Assets	2	2
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member]
Schedule of Equity Method Investments [Line Items]
Assets		0.1
Clean-Coal Investments [Member] | Controlling Interest [Member] | Twenty 2011 Era Clean Coal Plants [Member]
Schedule of Equity Method Investments [Line Items]
Assets	2.6	12.4
Other Investments [Member]
Schedule of Equity Method Investments [Line Items]
Assets	5.5	$ 3.9
Funding Commitments	$ 0.5

Derivatives and Hedging Activ_3

Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Detail) - USD ($)		Sep. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Notional Amount		$ 1,022,500,000	$ 598,600,000
Derivatives Assets		18,200,000	13,700,000
Derivative Liabilities		9,800,000	69,300,000
Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional Amount		1,000,000,000	550,000,000
Interest Rate Contracts [Member] | Other current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives Assets		5,900,000
Interest Rate Contracts [Member] | Other noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives Assets		5,900,000
Interest Rate Contracts [Member] | Accrued compensation and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities		300,000	54,500,000
Interest Rate Contracts [Member] | Other noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities		4,200,000	10,900,000
Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	22,500,000	48,600,000
Foreign Exchange Contracts [Member] | Other current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives Assets	[1]	4,500,000	5,900,000
Foreign Exchange Contracts [Member] | Other noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives Assets		1,900,000	7,800,000
Foreign Exchange Contracts [Member] | Accrued compensation and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	[1]	900,000	600,000
Foreign Exchange Contracts [Member] | Other noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities		$ 4,400,000	$ 3,300,000
[1]	Included within foreign exchange contracts at September 30, 2021 were $365.8 million of call options, offset with $365.8 million of put options, and $12.1 million of buy forwards, offset with $34.6 million of sell forwards.  Included within foreign exchange contracts at December 31, 2020 were $354.7 million of call options, offset with $354.7 million of put options, and $9.0 million of buy forwards, offset with $57.6 million of sell forwards.

Derivatives and Hedging Activ_4

Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Parenthetical) (Detail) - Foreign Exchange Contracts [Member] - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Call Options [Member]
Derivatives, Fair Value [Line Items]
Foreign exchange derivative contracts	$ 365.8	$ 354.7
Put Options [Member]
Derivatives, Fair Value [Line Items]
Foreign exchange derivative contracts	365.8	354.7
Forward Contracts [Member] | Call Options [Member]
Derivatives, Fair Value [Line Items]
Foreign exchange derivative contracts	34.6	57.6
Forward Contracts [Member] | Put Options [Member]
Derivatives, Fair Value [Line Items]
Foreign exchange derivative contracts	$ 12.1	$ 9

Derivatives and Hedging Activ_5

Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	$ 5.6	$ 10.4	$ 33.6	$ (115)
Amount of Loss Reclassified from Accumulated Other Comprehensive Loss into Earnings		(1)	(1.4)	(4.7)	(3.9)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.5	0.7	1.4	1.4
Interest expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	11.2	3.4	45.3	(104.7)
Amount of Loss Reclassified from Accumulated Other Comprehensive Loss into Earnings		(0.3)	(0.3)	(0.9)	(0.9)
Commission revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	(5.6)	7	(11.7)	(10.3)
Amount of Loss Reclassified from Accumulated Other Comprehensive Loss into Earnings		(0.4)	(0.4)	(2.7)	(0.9)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.2	(0.1)		(0.4)
Compensation expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Loss Reclassified from Accumulated Other Comprehensive Loss into Earnings		(0.1)	(0.4)	(0.6)	(1.2)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.2	0.4	0.8	1
Operating expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Loss Reclassified from Accumulated Other Comprehensive Loss into Earnings		(0.2)	(0.3)	(0.5)	(0.9)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		$ 0.1	$ 0.4	$ 0.6	$ 0.8
[1]	For the three and nine-month periods ended September 30, 2021, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of zero and $1.2 million, respectively.  For the three and nine-month periods ended September 30, 2020, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of $0.4 million and $0.8 million, respectively.

Derivatives and Hedging Activ_6

Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Other comprehensive income (loss), foreign currency transaction and translation adjustment, net of tax	$ (97.8)	$ 30.1	$ 58.6	$ 102.4	$ 249.9	$ (380.1)
Foreign Exchange [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other comprehensive income (loss), foreign currency transaction and translation adjustment, net of tax	$ 0			$ 0.4			$ 1.2	$ 0.8

Derivatives and Hedging Activ_7

Derivatives and Hedging Activity - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2021USD ($)
Scenario, Forecast [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Estimated pretax loss to be reclassified from accumulated other comprehensive loss into earnings	$ 8.1

Commitments, Contingencies an_3

Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail) $ in Millions	Sep. 30, 2021USD ($)
Senior Notes [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, Thereafter	$ 850
Contractual Obligations, Payments Due by Period, Total	850
Note Purchase Agreements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2022	200
Contractual Obligations, Payments Due by Period, 2023	250
Contractual Obligations, Payments Due by Period, 2024	475
Contractual Obligations, Payments Due by Period, 2025	200
Contractual Obligations, Payments Due by Period, Thereafter	3,323
Contractual Obligations, Payments Due by Period, Total	4,448
Premium Financing Debt Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	234.1
Contractual Obligations, Payments Due by Period, Total	234.1
Interest On Debt [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	59.9
Contractual Obligations, Payments Due by Period, 2022	217.7
Contractual Obligations, Payments Due by Period, 2023	208.7
Contractual Obligations, Payments Due by Period, 2024	192
Contractual Obligations, Payments Due by Period, 2025	177.4
Contractual Obligations, Payments Due by Period, Thereafter	1,406.9
Contractual Obligations, Payments Due by Period, Total	2,262.6
Total Debt Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	294
Contractual Obligations, Payments Due by Period, 2022	417.7
Contractual Obligations, Payments Due by Period, 2023	458.7
Contractual Obligations, Payments Due by Period, 2024	667
Contractual Obligations, Payments Due by Period, 2025	377.4
Contractual Obligations, Payments Due by Period, Thereafter	5,579.9
Contractual Obligations, Payments Due by Period, Total	7,794.7
Operating Lease Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	21.8
Contractual Obligations, Payments Due by Period, 2022	102
Contractual Obligations, Payments Due by Period, 2023	81.9
Contractual Obligations, Payments Due by Period, 2024	59.5
Contractual Obligations, Payments Due by Period, 2025	45.5
Contractual Obligations, Payments Due by Period, Thereafter	88.2
Contractual Obligations, Payments Due by Period, Total	398.9
Less Sublease Arrangements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Receivable by Period, 2021	(0.3)
Contractual Obligations, Payments Receivable by Period, 2022	(0.8)
Contractual Obligations, Payments Receivable by Period, 2023	(0.6)
Contractual Obligations, Payments Receivable by Period, 2024	(0.3)
Contractual Obligations, Payments Receivable by Period, 2025	(0.2)
Contractual Obligations, Payments Receivable by Period, Thereafter	(0.5)
Contractual Obligations, Payments Receivable by Period, Total	(2.7)
Outstanding Purchase Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	33.5
Contractual Obligations, Payments Due by Period, 2022	68.9
Contractual Obligations, Payments Due by Period, 2023	47.3
Contractual Obligations, Payments Due by Period, 2024	30.7
Contractual Obligations, Payments Due by Period, 2025	22.9
Contractual Obligations, Payments Due by Period, Thereafter	37.4
Contractual Obligations, Payments Due by Period, Total	240.7
Total Contractual Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2021	349
Contractual Obligations, Payments Due by Period, 2022	587.8
Contractual Obligations, Payments Due by Period, 2023	587.3
Contractual Obligations, Payments Due by Period, 2024	756.9
Contractual Obligations, Payments Due by Period, 2025	445.6
Contractual Obligations, Payments Due by Period, Thereafter	5,705
Contractual Obligations, Payments Due by Period, Total	$ 8,431.6

Commitments, Contingencies an_4

Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)	Jul. 29, 2021	Sep. 30, 2021USD ($)ft²EntityLetterOfCredit	Sep. 30, 2020Entity	Sep. 30, 2021USD ($)ft²EntityEmployeeLetterOfCredit	Sep. 30, 2020USD ($)Entity	Sep. 30, 2021USD ($)ft²EntityLetterOfCredit
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of square feet | ft²		360,000		360,000		360,000
Number of employees will accommodate at new facility | Employee				2,000
Number of companies acquired | Entity		44	55	80	126	602
Aggregate amount of maximum earnout obligations related to acquisitions				$ 1,026,100,000	$ 1,026,100,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet				578,700,000	$ 578,700,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option		$ 560,500,000		560,500,000		$ 560,500,000
Aggregate amount of earnout obligation expected settlement in cash		$ 18,200,000		18,200,000		$ 18,200,000
Income tax credits and adjustments				$ 108,000,000
Security Deposit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit		1		1		1
Collateral [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit		1		1		1
Self-Insurance Deductibles [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit		2		2		2
Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit		6		6		6
Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained				$ 2,000,000
Errors And Omissions [Member] | Up to $10 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained				15,000,000
Insurance claim arise during ordinary course of business		$ 15,000,000		15,000,000		$ 15,000,000
Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained				20,000,000
Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained				10,000,000
Letter of Credit [Member] | Security Deposit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt		500,000		500,000		500,000
Letter of Credit [Member] | Collateral [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt		900,000		900,000		900,000
Letter of Credit [Member] | Self-Insurance Deductibles [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt		9,400,000		9,400,000		9,400,000
Liabilities recorded on self-insurance		18,700,000		18,700,000		18,700,000
Letter of Credit [Member] | Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt		4,300,000		4,300,000		4,300,000
Minimum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value		2,800,000		2,800,000		2,800,000
Minimum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		15,000,000		15,000,000		15,000,000
Minimum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		100,000,000		100,000,000		100,000,000
Minimum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		225,000,000		225,000,000		225,000,000
Maximum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value		7,400,000		7,400,000		7,400,000
Maximum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		100,000,000		100,000,000		100,000,000
Maximum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		225,000,000		225,000,000		225,000,000
Maximum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business		$ 365,000,000		$ 365,000,000		$ 365,000,000
Unconsolidated Investments in Enterprises [Member] | Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest		1.00%		1.00%		1.00%
Unconsolidated Investments in Enterprises [Member] | Maximum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest		50.00%		50.00%		50.00%
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Notes redemption date	Aug. 13, 2021
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Notes redemption date	Aug. 13, 2021

Commitments, Contingencies an_5

Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2021	$ 0.8
Amount of Commitment Expiration by Period - 2022	1.4
Amount of Commitment Expiration by Period - 2023	1.5
Amount of Commitment Expiration by Period - 2024	2.3
Amount of Commitment Expiration by Period - 2025	0.2
Amount of Commitment Expiration by Period - Thereafter	15.3
Total Amounts Committed	21.5
Funding Commitments [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2021	0.5
Total Amounts Committed	0.5
Financial Guarantees [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2021	0.3
Amount of Commitment Expiration by Period - 2022	1.4
Amount of Commitment Expiration by Period - 2023	1.5
Amount of Commitment Expiration by Period - 2024	2.3
Amount of Commitment Expiration by Period - 2025	0.2
Amount of Commitment Expiration by Period - Thereafter	0.2
Total Amounts Committed	5.9
Letters of Credit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - Thereafter	15.1
Total Amounts Committed	$ 15.1

Supplemental Disclosures of C_3

Supplemental Disclosures of Cash Flow Information - Cash Flow (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash And Cash Equivalents [Abstract]
Interest paid	$ 150.4	$ 144.8
Income taxes paid, net	$ 235.2	$ 65.6

Supplemental Disclosures of C_4

Supplemental Disclosures of Cash Flow Information - Cash Equivalents And Restricted Cash (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 2,735.1	$ 664.6	$ 629.9
Restricted cash	3,277.2	$ 2,909.7	2,844.9
Total cash, cash equivalents and restricted cash	$ 6,012.3		$ 3,474.8

Supplemental Disclosures of C_5

Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - Qualified Contributory Savings and Thrift 401(k) Plan [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Matching contributions by employer, percentage		100.00%
Percentage of eligible compensation for matching contributions by employer	5.00%	5.00%
Matching contributions vesting schedule		5 years
Contribution expense to plan		$ 51.8	$ 45.9

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Loss Attributable to Controlling Interests (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance as of December 31, 2020	$ (643.6)
Balance as of September 30, 2021	(623.2)
Pension Liability [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance as of December 31, 2020	(56.1)
Net change in period	1.7
Balance as of September 30, 2021	(54.4)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance as of December 31, 2020	(491.1)
Net change in period	(9.7)
Balance as of September 30, 2021	(500.8)
Fair Value of Derivative Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance as of December 31, 2020	(96.4)
Net change in period	28.4
Balance as of September 30, 2021	(68)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance as of December 31, 2020	(643.6)
Net change in period	20.4
Balance as of September 30, 2021	$ (623.2)

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Loss - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Expense related to pension liability reclassified from accumulated other comprehensive earnings	$ 4,300,000	$ 4,600,000
Loss related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	4,700,000	3,900,000
Foreign currency translation reclassified from accumulated other comprehensive earnings	$ 0	$ 0

Segment Information - Additiona

Segment Information - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2021Segment
Segment Reporting [Abstract]
Number of reportable segments	3

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,138.2	$ 1,849.1	$ 6,236.7	$ 5,300
Earnings before income taxes	231.4	209.5	862.1	737.2
Identifiable assets at September 30, 2021 and 2020	26,915.8	21,814.9	26,915.8	21,814.9	$ 22,331.4
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Earnings before income taxes	334.5	276.4	1,114.5	935
Identifiable assets at September 30, 2021 and 2020	21,184.7	18,641.2	21,184.7	18,641.2
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	280.6	244.3	815	717
Earnings before income taxes	29.5	25.3	87	64.1
Identifiable assets at September 30, 2021 and 2020	1,079.7	964.5	1,079.7	964.5
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	357.9	310.2	921.6	651.7
Earnings before income taxes	(132.6)	(92.2)	(339.4)	(261.9)
Identifiable assets at September 30, 2021 and 2020	$ 4,651.4	$ 2,209.2	$ 4,651.4	$ 2,209.2

Segment Information - Summary o

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,105.6	$ 1,807.5	$ 6,135	$ 5,188.3
Reimbursements	32.6	41.6	101.7	111.7
Total revenues	2,138.2	1,849.1	6,236.7	5,300
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,499.7	1,294.6	4,500.1	3,931.3
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	248	202.7	713.3	605.3
Reimbursements	32.6	41.6	101.7	111.7
Total revenues	280.6	244.3	815	717
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	357.9	310.2	921.6	651.7
Total revenues	357.9	310.2	921.6	651.7
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,016.2	889.9	3,118.7	2,734.6
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,016.2	889.9	3,118.7	2,734.6
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	601.7	495.8	1,696.4	1,462.8
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	353.8	293.2	983.4	858.1
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	247.9	202.6	713	604.7
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	61	54.7	183	164
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	61	54.7	183	164
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.7	34.5	150.3	117
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.7	34.5	150.3	117
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	20.8	19	56	53.6
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	20.7	18.9	55.8	53
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1	0.1	0.2	0.6
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.3	3.4	9	4.6
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.3	3.4	8.9	4.6
Net Gains on Divestitures [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements			0.1
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	357.6	310.8	919.2	652.1
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	357.6	310.8	919.2	652.1
Other Net Revenue (Losses) [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	(0.6)	2.4	(0.4)
Other Net Revenue (Losses) [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.3	$ (0.6)	$ 2.4	$ (0.4)

Segment Information - Summary_2

Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,138.2	$ 1,849.1	$ 6,236.7	$ 5,300
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,592.1	1,380.7	4,513.8	3,867.2
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	288.6	240.6	930	775.6
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	92	82.8	278.7	229.1
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	69.7	58.5	219.9	175.4
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	44.4	40	130	112
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	51.4	46.5	164.3	140.7
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,000.3	866.6	2,919.3	2,613.3
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	276.5	230.8	895	746.2
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	62.4	56.6	187.1	156.6
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	68.1	57.1	215.5	171.5
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	41	37	118.9	103
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	51.4	46.5	164.3	140.7
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	280.6	244.3	815	717
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	233.9	203.9	672.9	602.2
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	12.1	9.8	35	29.4
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	29.6	26.2	91.6	72.5
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.6	1.4	4.4	3.9
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.4	3	11.1	9
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	357.9	310.2	921.6	651.7
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 357.9	$ 310.2	$ 921.6	$ 651.7