Argo Group International (ARGO-PA) 10-Q 9 Nov 21 2021 Q3 Quarterly report Financial data

Cover

Cover - shares	9 Months Ended
	Sep. 30, 2021	Nov. 04, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	1-15259
Entity Registrant Name	ARGO GROUP INTERNATIONAL HOLDINGS, LTD.
Entity Incorporation, State or Country Code	D0
Entity Tax Identification Number	98-0214719
Entity Address, Address Line One	90 Pitts Bay Road
Entity Address, City or Town	Pembroke
Entity Address, Postal Zip Code	HM08
Entity Address, Country	BM
City Area Code	441
Local Phone Number	296-5858
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		34,872,707
Entity Central Index Key	0001091748
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
Common Shares
Document Information [Line Items]
Title of 12(b) Security	Common Stock, par value of $1.00 per share
Trading Symbol	ARGO
Security Exchange Name	NYSE
Guarantee of Argo Group U.S., Inc. 6.500% Senior Notes due 2042
Document Information [Line Items]
Title of 12(b) Security	Guarantee of Argo Group U.S., Inc.  6.500% Senior Notes due 2042
Trading Symbol	ARGD
Security Exchange Name	NYSE
Depositary Shares
Document Information [Line Items]
Title of 12(b) Security	Depositary Shares, each representing a 1/1000th Interest in a shareof Series A 7.00% Non-Cumulative Preference Shares, par value $1.00 per share
Trading Symbol	ARGOPrA
Security Exchange Name	NYSE

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investments:
Fixed maturities available-for-sale, at fair value (cost: 2021 - $4,145.2, 2020 - $3,981.1; allowance for expected credit losses: 2021 - $7.0, 2020 - $6.6)	$ 4,197.3	$ 4,107.1
Equity securities, at fair value (cost: 2021 - $131.1; 2020 - $162.5)	181.8	176.7
Other investments (cost: 2021 - $425.8; 2020 - $429.4)	425.8	429.4
Short-term investments, at fair value (cost: 2021 - $507.8; 2020 - $541.4)	508.6	542.6
Total investments	5,313.5	5,255.8
Cash	202	148.8
Accrued investment income	20.9	21.8
Premiums receivable	701.9	679.8
Reinsurance recoverables	2,954	3,009
Goodwill	147.3	147.3
Intangible assets, net of accumulated amortization	60.5	60.5
Current income taxes receivable, net	3.8	3
Deferred tax asset, net	42.8	16.7
Deferred acquisition costs, net	175.7	163.6
Ceded unearned premiums	572.1	575.1
Operating lease right-of-use assets	87.7	82
Other assets	231.4	294.7
Assets held for sale	4.5	7.7
Total assets	10,518.1	10,465.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	5,439.5	5,406
Unearned premiums	1,540.2	1,464.8
Accrued underwriting expenses and other liabilities	255.2	167.6
Ceded reinsurance payable, net	763.8	950.4
Funds held	74.2	64.7
Senior unsecured fixed rate notes	140.3	140.2
Other indebtedness	58.7	60.7
Junior subordinated debentures	258.1	257.8
Operating lease liabilities	100.5	95.8
Total liabilities	8,630.5	8,608
Commitments and contingencies (Note 14)
Shareholders' equity:
Preferred shares and additional paid-in capital - $1.00 par, 30,000,000 shares authorized; 6,000 shares issued at September 30, 2021 and December 31, 2020, respectively; liquidation preference $25,000	144	144
Common shares - $1.00 par, 500,000,000 shares authorized; 46,178,719 and 46,009,966 shares issued at September 30, 2021 and December 31, 2020, respectively	46.2	46
Additional paid-in capital	1,385	1,380.2
Treasury shares (11,315,889 shares at September 30, 2021 and December 31, 2020, respectively)	(455.1)	(455.1)
Retained earnings	765.5	684.1
Accumulated other comprehensive income, net of taxes	2	58.6
Total shareholders' equity	1,887.6	1,857.8
Total liabilities and shareholders' equity	$ 10,518.1	$ 10,465.8

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, at cost	$ 4,145.2	$ 3,981.1
Allowance for expected credit losses	7	6.6
Equity securities, cost	131.1	162.5
Other investments, cost	425.8	429.4
Short-term investments, cost	$ 507.8	$ 541.4
Preferred shares, par value (in dollars per share)	$ 1	$ 1
Preferred shares, shares authorized (in shares)	30,000,000	30,000,000
Preferred shares, shares issued (in shares)	6,000	6,000
Liquidation preference (in dollars per share)	$ 25,000	$ 25,000
Common shares, par value (in dollars per share)	$ 1	$ 1
Common shares, shares authorized (in shares)	500,000,000	500,000,000
Common shares, shares issued (in shares)	46,178,719	46,009,966
Treasury shares (in shares)	11,315,889	11,315,889

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Premiums and other revenue:
Earned premiums	$ 487.5	$ 445.5	$ 1,423.9	$ 1,313.9
Net investment income	46.1	42	143.2	79
Net realized investment gains (losses):
Net realized investment gains (losses)	0.6	(5.7)	3.3	32.7
Change in fair value of equity securities	(5.1)	10.5	30.7	(13.7)
Credit losses on fixed maturity securities	(0.8)	(10.5)	(1.5)	(43)
Net realized investment (losses) gains	(5.3)	(5.7)	32.5	(24)
Total revenue	528.3	481.8	1,599.6	1,368.9
Expenses:
Losses and loss adjustment expenses	311.7	328.9	890.9	883
Underwriting, acquisition and insurance expenses	177.1	161.9	530.8	489.8
Non-operating expenses	8.2	3	20.9	9.8
Interest expense	5.5	6.9	16.3	21.7
Fee and other (income) expense, net	(1.1)	(0.8)	(1.8)	(2.7)
Foreign currency exchange (gains) losses	(1.3)	8.4	4.4	13.6
Total expenses	500.1	508.3	1,461.5	1,415.2
Income (loss) before income taxes	28.2	(26.5)	138.1	(46.3)
Income tax provision (benefit)	5.8	(3.4)	16.1	6.9
Net income (loss)	22.4	(23.1)	122	(53.2)
Dividends on preferred shares	2.6	2	7.9	2
Net Income (Loss) Available to Common Stockholders, Diluted, Total	19.8	(25.1)	114.1	(55.2)
Net Income (Loss) Available to Common Stockholders, Basic, Total	$ 19.8	$ (25.1)	$ 114.1	$ (55.2)
Net income (loss) attributable to common shareholders per common share:
Basic (in dollars per share)	$ 0.57	$ (0.72)	$ 3.28	$ (1.60)
Diluted (in dollars per share)	0.56	(0.72)	3.26	(1.60)
Cash dividend declared - common shares (in dollars per share)	$ 0.31	$ 0.31	$ 0.93	$ 0.93
Weighted average common shares:
Basic (in shares)	34,854,724	34,667,266	34,796,528	34,590,659
Diluted (in shares)	35,036,700	34,667,266	35,062,416	34,590,659

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 22.4	$ (23.1)	$ 122	$ (53.2)
Other comprehensive income (loss):
Foreign currency translation adjustments	(2.3)	(4.7)	0.2	(12.3)
Defined benefit pension plans:
Net gain arising during the year	0	0	1.9	0
Unrealized gains (losses) on fixed maturity securities:
(Losses) gains arising during the year	(31.5)	38.2	(60.3)	68.3
Reclassification adjustment for losses (gains) included in net income	8.2	4.5	(11)	(15.4)
Other comprehensive income (loss) before tax	(25.6)	38	(69.2)	40.6
Income tax provision related to other comprehensive income (loss):
Net gain arising during the year	0	0	0.4	0
(Losses) gains arising during the year	(4.8)	6.5	(11.1)	12.4
Reclassification adjustment for losses (gains) included in net income	0.4	0.5	(1.9)	(0.2)
Income tax (benefit) provision related to other comprehensive income (loss)	(4.4)	7	(12.6)	12.2
Other comprehensive income (loss), net of tax	(21.2)	31	(56.6)	28.4
Comprehensive income (loss)	$ 1.2	$ 7.9	$ 65.4	$ (24.8)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Preferred Shares and Additional Paid-in Capital	Common Shares	Additional Paid-In Capital	Treasury Shares	Retained Earnings	Retained EarningsCumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss)Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance at Dec. 31, 2019	$ 1,763.7	$ (2.2)	$ 0	$ 45.7	$ 1,376.6	$ (455.1)	$ 793.7	$ (7.9)	$ 2.8	$ 5.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(53.2)						(53.2)
Preferred shares issued	144		144
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	40.7								40.7
Other comprehensive loss, net - Other	(12.3)								(12.3)
Activity under stock incentive plans	6.8			0.4	6.4
Retirement of common shares (tax payments on equity compensation)	(6.6)			(0.1)	(6.5)
Employee stock purchase plan	2				2
Dividends on preferred shares	(2)						(2)
Cash dividend declared - common shares	(32.2)						(32.2)
Ending Balance at Sep. 30, 2020	1,848.7		144	46	1,378.5	(455.1)	698.4		36.9
Beginning Balance at Jun. 30, 2020	1,707.7		0	46	1,376.5	(455.1)	734.4		5.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(23.1)						(23.1)
Preferred shares issued	144		144
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	35.7								35.7
Other comprehensive loss, net - Other	(4.7)								(4.7)
Activity under stock incentive plans	1.5				1.5
Retirement of common shares (tax payments on equity compensation)	(0.1)				(0.1)
Employee stock purchase plan	0.6				0.6
Dividends on preferred shares	(2)						(2)
Cash dividend declared - common shares	(10.9)						(10.9)
Ending Balance at Sep. 30, 2020	1,848.7		144	46	1,378.5	(455.1)	698.4		36.9
Beginning Balance at Dec. 31, 2020	1,857.8		144	46	1,380.2	(455.1)	684.1		58.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	122						122
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	(58.3)								(58.3)
Other comprehensive loss, net - Other	1.7								1.7
Activity under stock incentive plans	6			0.2	5.8
Retirement of common shares (tax payments on equity compensation)	(2.5)				(2.5)
Employee stock purchase plan	1.5				1.5
Dividends on preferred shares	(7.9)						(7.9)
Cash dividend declared - common shares	(32.7)						(32.7)
Ending Balance at Sep. 30, 2021	1,887.6		144	46.2	1,385	(455.1)	765.5		2
Beginning Balance at Jun. 30, 2021	1,897.9		144	46.2	1,383.1	(455.1)	756.5		23.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	22.4						22.4
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	(18.9)								(18.9)
Other comprehensive loss, net - Other	(2.3)								(2.3)
Activity under stock incentive plans	1.5				1.5
Retirement of common shares (tax payments on equity compensation)	(0.1)				(0.1)
Employee stock purchase plan	0.5				0.5
Dividends on preferred shares	(2.6)						(2.6)
Cash dividend declared - common shares	(10.8)						(10.8)
Ending Balance at Sep. 30, 2021	$ 1,887.6		$ 144	$ 46.2	$ 1,385	$ (455.1)	$ 765.5		$ 2

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	Aug. 06, 2021	May 07, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Stockholders' Equity [Abstract]
Cash dividend declared - common shares (in dollars per share)	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.93	$ 0.93

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net income (loss)	$ 122	$ (53.2)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Amortization and depreciation	28.5	24.7
Share-based payments expense	6.4	7.7
Deferred income tax benefit, net	(14.4)	(16.5)
Net realized investment (gains) losses	(32.5)	24
Undistributed earnings from alternative investment portfolio	(74.8)	1.6
Loss on disposals of long-lived assets, net	1.7	0.4
Change in:
Accrued investment income	0.9	3.5
Receivables	30.2	(67.4)
Deferred acquisition costs	(12)	(11.7)
Ceded unearned premiums	2.3	(120.5)
Reserves for losses and loss adjustment expenses	36.5	267.6
Unearned premiums	77.3	176.8
Ceded reinsurance payable and funds held	(176.3)	(104.2)
Income taxes	(1)	(22.2)
Accrued underwriting expenses and other liabilities	42.5	(0.8)
Other, net	3.5	(8.4)
Cash provided by (used in) operating activities	40.8	101.4
Cash flows from investing activities:
Sales of fixed maturity investments	786.4	977.5
Maturities and mandatory calls of fixed maturity investments	591.1	410.3
Sales of equity securities	36.9	18.2
Sales of other investments	81.1	95.5
Purchases of fixed maturity investments	(1,511.9)	(1,659.3)
Purchases of equity securities	(5.3)	(68.3)
Purchases of other investments	(40.1)	(24.5)
Change in foreign regulatory deposits and voluntary pools	36.2	(0.5)
Change in short-term investments	32.3	297.8
Settlements of foreign currency exchange forward contracts	(9.7)	9.1
Proceeds from sale of Trident assets	0	38
Sale (Purchases) of fixed assets, net	16	(15.8)
Other, net	37.6	2.7
Cash (used in) provided by investing activities	50.6	80.7
Cash flows from financing activities:
Payment of long-term debt	0	(125)
Issuance of preferred shares, net of issuance costs	0	144
Activity under stock incentive plans	1.2	1.4
Payment of cash dividends to preferred shareholders	(7.9)	(2)
Payment of cash dividends to common shareholders	(32.7)	(32.2)
Cash used in financing activities	(39.4)	(13.8)
Effect of exchange rate changes on cash	1.2	(5.7)
Change in cash	53.2	162.6
Cash, beginning of year	148.8	137.8
Cash, end of period	$ 202	$ 300.4

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. and its subsidiaries (“Argo Group,” “Argo,” “we,” “us,” “our” or the “Company”) have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Argo Group is an underwriter of specialty insurance products in the property and casualty market. The preparation of interim financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for expected credit losses; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment, including the allowance for credit losses on fixed maturity securities; valuation of goodwill and intangibles and our deferred tax asset valuation allowance. Actual results could materially differ from those estimates. Certain financial information that is normally included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2020, filed with the Securities and Exchange Commission ("SEC") on March 15, 2021. The interim financial information as of, and for, the three and nine months ended, September 30, 2021 and 2020 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation. Certain reclassifications have been made to financial information presented for prior years to conform to the current year’s presentation. Update on Trident Transaction On April 30, 2020, we sold our Trident Public Risk Solutions (“Trident”) brand and underwriting platform. During the second quarter of 2021, the parties to the transaction agreed to amend the transaction and other related agreements associated with the sale of Trident to eliminate certain lines of business from the transaction. As a result, we recognized a $16.5 million reduction in the original gain on sale during the second quarter of 2021. Additionally, we recognized a $5.0 million gain during the second quarter of 2021 related to meeting certain post-closing performance conditions. This transaction is included in "Net realized investment gains (losses)" in our Consolidated Statements of Income (Loss) for the nine months ended September 30, 2021.

Recently Issued Accounting Pron

Recently Issued Accounting Pronouncements	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Recently Issued Accounting Pronouncements	Recently Issued Accounting PronouncementsThe Company has evaluated recently issued accounting pronouncements and determined none are material to our results of operations or financial position reported herein.

Investments

Investments	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Investments	InvestmentsIncluded in “Total investments” in our Consolidated Balance Sheets at September 30, 2021 and December 31, 2020 is $97.1 million and $140.3 million, respectively, of assets managed on behalf of the trade capital providers, who are third-party participants that provide underwriting capital to the operations of Syndicates 1200 and 1910. Fixed Maturities The amortized cost, gross unrealized gains, gross unrealized losses, allowance for credit losses, and fair value of fixed maturity investments were as follows: September 30, 2021 (in millions) Amortized Gross Gross Allowance for Credit Losses Fair Fixed maturities U.S. Governments $ 404.8 $ 7.5 $ 2.1 $ — $ 410.2 Foreign Governments 229.3 2.7 2.7 0.2 229.1 Obligations of states and political subdivisions 169.9 6.7 0.9 — 175.7 Corporate bonds 1,996.2 47.8 14.4 6.6 2,023.0 Commercial mortgage-backed securities 403.1 9.1 3.0 — 409.2 Residential mortgage-backed securities 488.1 10.6 4.8 — 493.9 Asset-backed securities 110.9 2.2 0.1 0.2 112.8 Collateralized loan obligations 342.9 1.8 1.3 — 343.4 Total fixed maturities $ 4,145.2 $ 88.4 $ 29.3 $ 7.0 $ 4,197.3 December 31, 2020 (in millions) Amortized Gross Gross Allowance for Credit Losses Fair Fixed maturities U.S. Governments $ 385.4 $ 14.7 $ 0.3 $ — $ 399.8 Foreign Governments 284.1 11.6 0.7 0.2 294.8 Obligations of states and political subdivisions 163.1 7.7 0.3 0.1 170.4 Corporate bonds 1,925.9 75.3 13.3 6.1 1,981.8 Commercial mortgage-backed securities 324.8 15.2 0.3 — 339.7 Residential mortgage-backed securities 491.4 17.4 0.6 — 508.2 Asset-backed securities 120.5 2.9 0.4 0.2 122.8 Collateralized loan obligations 285.9 4.9 1.2 — 289.6 Total fixed maturities $ 3,981.1 $ 149.7 $ 17.1 $ 6.6 $ 4,107.1 Contractual Maturity The amortized cost and fair values of fixed maturity investments as of September 30, 2021, by contractual maturity, were as follows: (in millions) Amortized Fair Due in one year or less $ 288.7 $ 292.9 Due after one year through five years 1,670.6 1,695.1 Due after five years through ten years 719.6 728.6 Due after ten years 121.3 121.4 Structured securities 1,345.0 1,359.3 Total $ 4,145.2 $ 4,197.3 The expected maturities may differ from the contractual maturities because debtors may have the right to call or prepay obligations. Other Investments Details regarding the carrying value and unfunded investment commitments of other investments as of September 30, 2021 and December 31, 2020 were as follows: September 30, 2021 (in millions) Carrying Unfunded Investment Type Hedge funds $ 102.1 $ — Private equity 254.3 63.0 Overseas deposits 64.6 — Other 4.8 — Total other investments $ 425.8 $ 63.0 December 31, 2020 (in millions) Carrying Unfunded Investment Type Hedge funds $ 111.2 $ — Private equity 211.4 80.0 Overseas deposits 102.1 — Other 4.7 — Total other investments $ 429.4 $ 80.0 The following describes each investment type: • Hedge funds: Hedge funds include funds that primarily buy and sell stocks, including short sales, multi-strategy credit, relative value credit and distressed credit. • Private equity: Private equity includes buyout funds, real asset/infrastructure funds, credit special situations funds, mezzanine lending funds and direct investments and strategic non-controlling minority investments in private companies that are principally accounted for using the equity method of accounting. • Overseas deposits: Overseas deposits are principally invested in short-term sovereign fixed income and investment grade corporate securities and international stocks. • Other: Other includes participation in investment pools. Unrealized Losses An aging of unrealized losses on our investments in fixed maturities is presented below: September 30, 2021 Less Than One Year One Year or Greater Total (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Fixed maturities U.S. Governments $ 161.1 $ 1.7 $ 5.6 $ 0.4 $ 166.7 $ 2.1 Foreign Governments 117.7 2.4 1.3 0.3 119.0 2.7 Obligations of states and political subdivisions 28.3 0.5 0.4 0.4 28.7 0.9 Corporate bonds 626.4 10.8 40.7 3.6 667.1 14.4 Commercial mortgage-backed securities 136.9 2.6 8.9 0.4 145.8 3.0 Residential mortgage-backed securities 207.4 4.6 4.8 0.2 212.2 4.8 Asset-backed securities 21.3 0.1 — — 21.3 0.1 Collateralized loan obligations 126.8 1.3 2.4 — 129.2 1.3 Total fixed maturities $ 1,425.9 $ 24.0 $ 64.1 $ 5.3 $ 1,490.0 $ 29.3 December 31, 2020 Less Than One Year One Year or Greater Total (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Fixed maturities U.S. Governments $ 40.6 $ 0.3 $ — $ — $ 40.6 $ 0.3 Foreign Governments 18.0 0.5 0.1 0.2 18.1 0.7 Obligations of states and political subdivisions 5.2 0.3 — — 5.2 0.3 Corporate bonds 202.5 6.7 17.5 6.6 220.0 13.3 Commercial mortgage-backed securities 21.8 0.3 — — 21.8 0.3 Residential mortgage-backed securities 74.4 0.4 3.0 0.2 77.4 0.6 Asset-backed securities 4.6 0.4 — — 4.6 0.4 Collateralized loan obligations 121.1 0.9 49.1 0.3 170.2 1.2 Total fixed maturities $ 488.2 $ 9.8 $ 69.7 $ 7.3 $ 557.9 $ 17.1 We hold a total of 5,377 fixed maturity securities, of which 577 were in an unrealized loss position for less than one year and 85 were in an unrealized loss position for a period one year or greater as of September 30, 2021. Allowance for Credit Losses For fixed maturities with a decline in the fair value between the amortized cost due to credit-related factors, an allowance is established for the difference between the estimated recoverable value and amortized cost with a corresponding charge to realized investment losses in the Statement of Income (Loss). The allowance is limited to the difference between amortized cost and fair value. The estimated recoverable value is the present value of cash flows expected to be collected, as determined by management. The difference between fair value and amortized cost that is not associated with credit-related factors is recognized in the Statement of Comprehensive Income (Loss). Accrued interest is excluded from the measurement of the allowance for credit losses. When determining if a credit loss has been incurred, we may consider the historical performance of the security, available market information and security specific considerations such as the priority payment of the security. In addition, inputs used in our analysis include, but are not limited to, credit ratings and downgrades, delinquency rates, missed scheduled interest or principal payments, purchase yields, underlying asset performance, collateral types, modeled default rates, modeled severity rates, call/prepayment rates, expected cash flows, industry concentrations, and potential or filed bankruptcies or restructurings. We evaluate for credit losses each quarter. If we determine that all or a portion of a fixed maturity is uncollectible, the uncollectible amortized cost is written off with a corresponding reduction to the allowance for credit losses. If we collect cash flows that were previously written off, the recovery is recognized in realized investment gains. We also consider whether we intend to sell an available-for-sale security or if it is more likely than not that we will be required to sell the security before recovery of its amortized cost. In these instances, a decline in fair value is recognized in net realized gains (losses) in the Statement of Income based on the fair value of the security at the time of assessment, resulting in a new cost basis for the security. The following table presents a roll-forward of the changes in allowance for credit losses on available-for-sale fixed maturities by industry category for the three and nine months ending September 30, 2021 and 2020, respectively: (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, June 30, 2021 $ 0.2 $ — $ 6.2 $ — $ 6.4 Securities for which allowance was not previously recorded — — 0.7 0.2 0.9 Securities sold during the period — — (0.2) — (0.2) Additional net increases (decreases) in existing allowance — — (0.1) — (0.1) Ending balance, September 30, 2021 $ 0.2 $ — $ 6.6 $ 0.2 $ 7.0 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, June 30, 2020 $ 0.3 $ 0.1 $ 39.7 $ 0.1 $ 40.2 Securities for which allowance was not previously recorded 0.1 — 13.6 — 13.7 Securities sold during the period (0.2) — (14.7) — (14.9) Additional net increases (decreases) in existing allowance — 0.1 (6.4) 1.0 (5.3) Ending balance, September 30, 2020 $ 0.2 $ 0.2 $ 32.2 $ 1.1 $ 33.7 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, January 1, 2021 $ 0.2 $ 0.1 $ 6.1 $ 0.2 $ 6.6 Securities for which allowance was not previously recorded — — 2.7 0.2 2.9 Securities sold during the period — — (1.1) — (1.1) Additional net increases (decreases) in existing allowance — (0.1) (1.1) (0.2) (1.4) Ending balance, September 30, 2021 $ 0.2 $ — $ 6.6 $ 0.2 $ 7.0 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, January 1, 2020 $ — $ — $ — $ — $ — Additions-initial adoption of accounting standard — — 6.8 0.1 6.9 Securities for which allowance was not previously recorded 0.3 0.3 14.4 — 15.0 Securities sold during the period (0.2) — (15.5) — (15.7) Additional net increases (decreases) in existing allowance 0.1 (0.1) 26.5 1.0 27.5 Ending balance, September 30, 2020 $ 0.2 $ 0.2 $ 32.2 $ 1.1 $ 33.7 Total credit impairment (gains) losses included in net realized investment gains (losses) in the Consolidated Statement of Income was $0.8 million and $1.5 million for the three and nine months ended September 30, 2021, respectively. Total credit impairment losses included in net realized investment gains (losses) in the Consolidated Statement of Income was $10.5 million and $43.0 million for the three and nine months ended September 30, 2020, respectively. Investment Gains and Losses The following table presents our gross realized investment gains (losses): For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Realized gains on fixed maturities and other Fixed maturities $ 3.3 $ 7.2 $ 26.3 $ 30.5 Other investments, including short-terms 0.9 10.8 8.9 70.7 Other assets — — 3.4 32.3 4.2 18.0 38.6 133.5 Realized losses on fixed maturities and other Fixed maturities (0.4) (7.5) (5.0) (25.7) Other investments, including short-terms (3.3) (12.6) (16.6) (58.4) Other assets (1.4) (0.4) (14.0) (0.9) Credit losses on fixed maturities (0.7) (10.5) (1.5) (43.0) (5.8) (31.0) (37.1) (128.0) Equity securities Net realized gains (losses) on equity securities 1.4 (3.2) 0.3 (15.8) Change in unrealized gains (losses) on equity securities held at the end of the period (5.1) 10.5 30.7 (13.7) Net realized gains (losses) on equity securities (3.7) 7.3 31.0 (29.5) Net realized investment and other gains (losses) before income taxes (5.3) (5.7) 32.5 (24.0) Income tax (benefit) provision — (0.5) — (1.7) Net realized investment gains (losses), net of income taxes $ (5.3) $ (5.2) $ 32.5 $ (22.3) The cost of securities sold is based on the specific identification method. Changes in unrealized gains (losses) related to investments are summarized as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Change in unrealized gains (losses) Fixed maturities $ (23.0) $ 42.5 $ (70.7) $ 66.9 Other investments — 0.2 — (14.0) Other and short-term investments (0.3) — (0.6) — Net unrealized investment gains (losses) before income taxes (23.3) 42.7 (71.3) 52.9 Income tax provision (benefit) (4.4) 7.0 (13.0) 12.2 Net unrealized investment gains (losses), net of income taxes $ (18.9) $ 35.7 $ (58.3) $ 40.7 Foreign Currency Exchange Forward Contracts We enter into foreign currency exchange forward contracts to manage operational currency exposure on our Canadian dollar investment portfolio and certain catastrophic events, minimize negative impacts to investment portfolio returns and gain exposure to a total return strategy which invests in multiple currencies. The currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other liabilities” and “Other assets” at September 30, 2021 and December 31, 2020, respectively. The net realized gains and (losses) are included in “Net realized investment gains (losses)” in our Consolidated Statements of Income (Loss). The fair value of our foreign currency exchange forward contracts as of September 30, 2021 and December 31, 2020 was as follows: (in millions) September 30, 2021 December 31, 2020 Operational currency exposure $ (0.3) $ 0.4 Asset manager investment exposure 2.0 (0.2) Total return strategy (3.4) 0.7 Total $ (1.7) $ 0.9 The following table represents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Realized gains Operational currency exposure $ — $ 4.2 $ 7.1 $ 10.1 Asset manager investment exposure 1.0 — 2.8 1.0 Total return strategy 6.9 10.3 13.0 43.5 Gross realized investment gains 7.9 14.5 22.9 54.6 Realized losses Operational currency exposure (5.2) (1.8) (17.8) (6.5) Asset manager investment exposure — (0.9) (0.8) (1.9) Total return strategy (6.9) (10.4) (12.0) (47.0) Gross realized investment losses (12.1) (13.1) (30.6) (55.4) Net realized investment (losses) gains on foreign currency exchange forward contracts $ (4.2) $ 1.4 $ (7.7) $ (0.8) Regulatory Deposits, Pledged Securities and Letters of Credit We are required to maintain assets on deposit with various regulatory authorities to support our insurance and reinsurance operations. We maintain assets pledged as collateral in support of irrevocable letters of credit issued under the terms of certain reinsurance agreements for reported loss and loss expense reserves. The following table presents our components of restricted assets: (in millions) September 30, 2021 December 31, 2020 Securities on deposit for regulatory and other purposes $ 205.3 $ 227.5 Securities pledged as collateral for letters of credit and other 191.0 189.4 Securities and cash on deposit supporting Lloyd’s business (1) 331.2 409.2 Total restricted investments $ 727.5 $ 826.1 (1) As the corporate member for our Lloyd’s syndicates, Argo’s share of Argo (No. 604) Limited’s required Funds to maintain at Lloyd’s was $397.9 million at September 30, 2021, of which $179.0 million was provided by Argo Re, Ltd. (“Argo Re.”) Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability, or in the absence of a principal market, the most advantageous market. Market participants are buyers and sellers in the principal (or most advantageous) market that are independent, knowledgeable, able to transact for the asset or liability and willing to transfer the asset or liability. Valuation techniques consistent with the market approach, income approach and/or cost approach are used to measure fair value. The inputs of these valuation techniques are categorized into three levels. • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that can be accessed at the reporting date. We define actively traded as a security that has traded in the past seven days. We receive one quote per instrument for Level 1 inputs. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. We receive one quote per instrument for Level 2 inputs. • Level 3 inputs are unobservable inputs. Unobservable inputs reflect our own judgments about the assumptions market participants would use in pricing the asset or liability based on the best information available in the circumstances. We receive fair value prices from third-party pricing services and our outside investment managers. These prices are determined using observable market information such as dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. We have reviewed the processes used by the third-party providers for pricing the securities and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of September 30, 2021 and December 31, 2020. A description of the valuation techniques we use to measure assets at fair value is as follows: Fixed Maturities (Available-for-Sale) Levels 1 and 2: • United States Treasury securities are typically valued using Level 1 inputs. For these securities, we obtain fair value measurements from third-party pricing services using quoted prices (unadjusted) in active markets at the reporting date. • United States Government agencies, non-U.S. Government securities, obligations of states and political subdivisions, credit securities and foreign denominated government and credit securities are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, yield curves, live trading levels, trade execution data, credit information and the security’s terms and conditions, among other things. • Asset and mortgage-backed securities and collateralized loan obligations are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. Fixed Maturities (Available-for-Sale) Levels 3: We own term loans that are valued using unobservable inputs. Equity Securities Level 1: Equity securities are principally reported at fair value using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Equity Securities Level 3: We own certain equity securities that are reported at fair value using Level 3 inputs. The valuation techniques for these securities include the following: • Fair value measurements for an investment in an equity fund obtained by applying final prices provided by the administrator of the fund, which is based upon certain estimates and assumptions. • Fair value measurements from brokers and independent valuation services, both based upon estimates, assumptions and other unobservable inputs. Other Investments Level 2: Foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. These assets are invested in short-term government securities, agency securities and corporate bonds and are valued using Level 2 inputs based upon values obtained from Lloyd’s. Short-term Investments: Short-term investments are principally reported at fair value using Level 1 inputs, with the exception of short-term corporate and governmental bonds reported at fair value using Level 2 inputs as described in the fixed maturities section above. Values for the investments categorized as Level 1 are obtained from various financial institutions as of the reporting date. Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair Value Measurements at Reporting Date Using (in millions) September 30, Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 410.2 $ 398.3 $ 11.9 $ — Foreign Governments 229.1 — 229.1 — Obligations of states and political subdivisions 175.7 — 175.7 — Corporate bonds 2,023.0 — 2,019.2 3.8 Commercial mortgage-backed securities 409.2 — 409.2 — Residential mortgage-backed securities 493.9 — 493.9 — Asset-backed securities 112.8 — 112.8 — Collateralized loan obligations 343.4 — 343.4 — Total fixed maturities 4,197.3 398.3 3,795.2 3.8 Equity securities 181.8 156.3 — 25.5 Other investments 65.1 — 65.1 — Short-term investments 508.6 497.4 11.2 — $ 4,952.8 $ 1,052.0 $ 3,871.5 $ 29.3 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 399.8 $ 383.5 $ 16.3 $ — Foreign Governments 294.8 — 294.8 — Obligations of states and political subdivisions 170.4 — 170.4 — Corporate bonds 1,981.8 — 1,974.8 7.0 Commercial mortgage-backed securities 339.7 — 339.7 — Residential mortgage-backed securities 508.2 — 508.2 — Asset-backed securities 122.8 — 122.8 — Collateralized loan obligations 289.6 — 289.6 — Total fixed maturities 4,107.1 383.5 3,716.6 7.0 Equity securities 176.7 159.2 — 17.5 Other investments 102.5 0.4 102.1 — Short-term investments 542.6 526.5 16.1 — $ 4,928.9 $ 1,069.6 $ 3,834.8 $ 24.5 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs The fair value measurements in the tables above do not equal “Total investments” on our Consolidated Balance Sheets as they exclude certain other investments that are accounted for under the equity-method of accounting. A reconciliation of the beginning and ending balances for the investments categorized as Level 3 are as follows: Fair Value Measurements Using Observable Inputs (Level 3) (in millions) Corporate Bonds Equity Short Term Investments Total Beginning balance, January 1, 2021 $ 7.0 $ 17.5 $ — $ 24.5 Transfers into Level 3 — 1.5 — 1.5 Transfers out of Level 3 — — — — Total gains or losses (realized/unrealized): Included in net income 0.1 6.5 — 6.6 Included in other comprehensive income 0.1 — — 0.1 Purchases, issuances, sales, and settlements: Purchases 0.1 — — 0.1 Issuances — — — — Sales (3.5) — — (3.5) Settlements — — — — Ending balance, September 30, 2021 $ 3.8 $ 25.5 $ — $ 29.3 Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at September 30, 2021 $ — $ — $ — $ — (in millions) Credit Financial Equity Total Beginning balance, January 1, 2020 $ 7.4 $ 18.2 $ 25.6 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income — (5.9) (5.9) Included in other comprehensive loss (0.5) — (0.5) Purchases, issuances, sales, and settlements: Purchases 0.1 5.2 5.3 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2020 $ 7.0 $ 17.5 $ 24.5 Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2020 $ — $ — $ — At September 30, 2021 and December 31, 2020, we did not have any financial assets or financial liabilities measured at fair value on a nonrecurring basis or any financial liabilities on a recurring basis.

Allowance for Credit Losses

Allowance for Credit Losses	9 Months Ended
	Sep. 30, 2021
Credit Loss [Abstract]
Allowance for Credit Losses	Allowance for Credit Losses Premiums receivable The following table represents the balances of premiums receivable, net of allowance for expected credit losses, at September 30, 2021 and January 1, 2021, and the changes in the allowance for expected credit losses for the nine months ended September 30, 2021. (in millions) Premiums Receivable, Net of Allowance for Estimated Uncollectible Premiums Allowance for Estimated Uncollectible Premiums Balance, January 1, 2021 $ 679.8 $ 9.4 Current period change for estimated uncollectible premiums — Write-offs of uncollectible premiums receivable (0.6) Foreign exchange adjustments — Balance, September 30, 2021 $ 701.9 $ 8.8 Reinsurance Recoverables The following table presents the balances of reinsurance recoverables, net of the allowance for estimated uncollectible reinsurance, at September 30, 2021 and January 1, 2021, and changes in the allowance for estimated uncollectible reinsurance for the nine months ended September 30, 2021. (in millions) Reinsurance Recoverables, Net of Allowance for Estimated Uncollectible Reinsurance Allowance for Estimated Uncollectible Reinsurance Balance, January 1, 2021 $ 3,009.0 $ 4.1 Current period change for estimated uncollectible reinsurance (0.3) Write-offs of uncollectible reinsurance recoverables — Balance, September 30, 2021 $ 2,954.0 $ 3.8 We primarily utilize A.M. Best credit ratings when determining the allowance, adjusted as needed based on our historical experience with the reinsurers. A portion of our reinsurance recoverables are collateralized by letters of credit, funds held or trust agreements.

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2021
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Nine Months Ended (in millions) 2021 2020 Net reserves beginning of the year $ 2,906.1 $ 2,722.7 Net AIL reserves acquired — 27.9 Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 884.9 876.9 Prior accident years 6.0 6.1 Losses and LAE incurred during calendar year, net of reinsurance 890.9 883.0 Deduct: Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior (1) 219.7 — Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 100.5 188.2 Prior accident years 552.8 632.7 Losses and LAE payments made during current calendar year, net of reinsurance: 653.3 820.9 Change in participation interest (2) 12.5 33.0 Foreign exchange adjustments (7.3) 23.0 Net reserves - end of period 2,929.2 2,868.7 Add: Reinsurance recoverables on unpaid losses and LAE, end of period 2,510.3 2,521.3 Gross reserves - end of period $ 5,439.5 $ 5,390.0 (1) Amount represents reserves ceded under the reinsurance to close transaction with RiverStone for Lloyd’s years of account 2017 and prior, effective January 1, 2020. (2) Amount represents the change in reserves due to changing our participation in Syndicates 1200 and 1910. Reserves for losses and LAE represent the estimated indemnity cost and related adjustment expenses necessary to investigate and settle claims. Such estimates are based upon individual case estimates for reported claims, estimates from ceding companies for reinsurance assumed and actuarial estimates for losses that have been incurred but not yet reported to the insurer. Any change in probable ultimate liabilities is reflected in current operating results. The impact from the (favorable) unfavorable development of prior accident years’ loss and LAE reserves on each reporting segment is presented below: For the Nine Months Ended (in millions) 2021 2020 U.S. Operations $ (0.7) $ (0.5) International Operations 0.1 (4.5) Run-off Lines 6.6 11.1 Total (favorable) unfavorable prior-year development $ 6.0 $ 6.1 The following describes the primary factors behind each segment’s prior accident year reserve development for the nine months ended September 30, 2021 and 2020: Nine months ended September 30, 2021: • U.S. Operations : Favorable development primarily in specialty lines, partially offset by unfavorable development in liability and professional lines. • International Operations : Unfavorable development primarily related to a one-time accounting adjustment and large claim movements in Argo Insurance Bermuda, partially offset by favorable development in property lines, including losses associated with prior year catastrophe losses. • Run-off Lines: Unfavorable loss reserve development on prior accident years in risk management workers compensation, other run-off lines and an individual environmental loss. Nine months ended September 30, 2020: • U.S. Operations: Favorable development in surety and commercial multi-peril, partially offset by unfavorable development in general liability, special property and commercial auto liability. • International Operations: Favorable development in property, specialty and reinsurance, partially offset by unfavorable development in general liability and surety lines. • Run-off Lines: Unfavorable loss reserve development on prior accident years in asbestos and environmental, driven by asbestos claims remaining open longer than expected, higher defense costs and movement on individual environmental claims as well as unfavorable development in other run-off lines, partially offset by favorable development in risk management workers compensation. In the opinion of management, our reserves represent the best estimate of our ultimate liabilities, based on currently known facts, current law, current technology and reasonable assumptions where facts are not known. Due to the significant uncertainties and related management judgments, there can be no assurance that future favorable or unfavorable loss development, which may be material, will not occur.

Disclosures about Fair Value of

Disclosures about Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Disclosures about Fair Value of Financial Instruments	Disclosures about Fair Value of Financial Instruments Cash. The carrying amount approximates fair value. Investment securities and short-term investments. See Note 3, “Investments,” for additional information. Premiums receivable and reinsurance recoverables on paid losses. The carrying value of current receivables and reinsurance recoverables on paid losses approximates fair value. Debt. At September 30, 2021 and December 31, 2020, the fair value of our debt instruments is determined using both Level 1 and Level 2 inputs, as previously defined in Note 3, “Investments.” We receive fair value prices from third-party pricing services for our financial instruments as well as for similar financial instruments. These prices are determined using observable market information such as publicly traded quoted prices, and trading prices for similar financial instruments actively being traded in the current market. We have reviewed the processes used by the third-party providers for pricing the securities and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of September 30, 2021 and December 31, 2020. A description of the valuation techniques we use to measure these liabilities at fair value is as follows: Senior Unsecured Fixed Rate Notes Level 1: • Our senior unsecured fixed rate notes are valued using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Junior Subordinated Debentures and Floating Rate Loan Stock Level 2: • Our trust preferred debentures, subordinated debentures and floating rate loan stock are typically valued using Level 2 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices for similar securities being traded in active markets at the reporting date, as our specific debt instruments are more infrequently traded. A summary of our financial instruments whose carrying value did not equal fair value is shown below: September 30, 2021 December 31, 2020 (in millions) Carrying Fair Carrying Fair Junior subordinated debentures: Trust preferred debentures $ 172.7 $ 173.1 $ 172.7 $ 173.6 Subordinated debentures 85.4 92.0 85.1 92.3 Total junior subordinated debentures 258.1 265.1 257.8 265.9 Senior unsecured fixed rate notes 140.3 150.1 140.2 146.7 Floating rate loan stock 58.7 58.9 60.7 61.0 457.1 474.1 458.7 473.6 Based on an analysis of the inputs, our financial instruments measured at fair value for disclosure purposes have been categorized as follows: Fair Value Measurements at Reporting Date Using (in millions) September 30, 2021 Level 1 (a) Level 2 (b) Level 3 (c) Junior subordinated debentures: Trust preferred debentures $ 173.1 $ — $ 173.1 $ — Subordinated debentures 92.0 — 92.0 — Total junior subordinated debentures 265.1 — 265.1 — Senior unsecured fixed rate notes 150.1 150.1 — — Floating rate loan stock 58.9 — 58.9 — 474.1 150.1 324.0 — (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2020 Level 1 (a) Level 2 (b) Level 3 (c) Junior subordinated debentures: Trust preferred debentures $ 173.6 $ — $ 173.6 $ — Subordinated debentures 92.3 — 92.3 — Total junior subordinated debentures 265.9 — 265.9 — Senior unsecured fixed rate notes 146.7 146.7 — — Floating rate loan stock 61.0 — 61.0 — 473.6 146.7 326.9 — (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs

Shareholders' Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Shareholders' Equity	Shareholders’ Equity On August 6, 2021, our Board of Directors declared a quarterly cash dividend in the amount of $0.31 on each common share outstanding. On September 15, 2021, we paid $10.8 million to our shareholders of record on August 31, 2021. On August 6, 2021, our Board of Directors declared a quarterly cash dividend in the amount of $437.500 per share on our 7.00% Resettable Fixed Rate Preference Shares, Series A, par value of $1.00 per share, with a liquidation preference of $25,000 per share (the “Series A Preference Shares”). Holders of depositary shares, each representing a 1/1,000th interest in a Series A Preference Share (the “Depositary Shares”), received $0.437500 per Depositary Share. On September 15, 2021, we paid $2.6 million to our shareholders of record of Series A Preference Shares on August 31, 2021. On August 7, 2020, our Board of Directors declared a quarterly cash dividend in the amount of $0.31 on each common share outstanding. On September 11, 2020, we paid $10.9 million to our shareholders of record on August 28, 2020. On August 7, 2020, our Board of Directors declared a quarterly cash dividend in the amount of $330.556 per share on our 7.00% Resettable Fixed Rate Preference Shares, Series A, par value of $1.00 per share, with a liquidation preference of $25,000 per share (the “Series A Preference Shares”). Holders of depositary shares, each representing a 1/1,000th interest in a Series A Preference Share (the “Depositary Shares”), received $0.330556 per Depositary Share. On September 15, 2020, we paid $2.0 million to our shareholders of record of Series A Preference Shares on September 1, 2020. On May 3, 2016, our Board of Directors authorized the repurchase of up to $150.0 million of our common shares (“2016 Repurchase Authorization”). The 2016 Repurchase Authorization supersedes all previous repurchase authorizations. As of September 30, 2021, availability under the 2016 Repurchase Authorization for future repurchases of our common shares was $53.3 million. We did not repurchase any common shares for the nine months ended September 30, 2021.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) A summary of changes in accumulated other comprehensive (loss) income, net of taxes (where applicable) by component for the nine months ended September 30, 2021 and 2020 is presented below: (in millions) Foreign Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2021 $ (37.9) $ 105.1 $ (8.6) $ 58.6 Other comprehensive income (loss) before reclassifications 0.2 (49.2) 1.5 (47.5) Amounts reclassified from accumulated other comprehensive loss — (9.1) — (9.1) Net current-period other comprehensive income (loss) 0.2 (58.3) 1.5 (56.6) Balance, September 30, 2021 $ (37.7) $ 46.8 $ (7.1) $ 2.0 (in millions) Foreign Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2020 $ (22.6) $ 33.5 $ (8.1) $ 2.8 Other comprehensive income (loss) before reclassifications (12.3) 55.9 — 43.6 Amounts reclassified from accumulated other comprehensive loss — (15.2) — (15.2) Net current-period other comprehensive income (loss) (12.3) 40.7 — 28.4 Cumulative effect of adoption of ASU 2016-13 — 5.7 — 5.7 Balance, September 30, 2020 $ (34.9) $ 79.9 $ (8.1) $ 36.9 The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income (Loss): For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Unrealized gains and losses on securities: Net realized investment losses (gains) $ 8.2 $ 4.5 $ (11.0) $ (15.4) (Benefit) provision for income taxes (0.4) (0.5) 1.9 0.2 Net of taxes $ 7.8 $ 4.0 $ (9.1) $ (15.2) Income tax effects are released from accumulated other comprehensive income (loss) for unrealized gains or losses when the gains or losses are realized, and are taxed at the statutory rate based on jurisdiction of the underlying transaction.

Net Income (Loss) Per Common Sh

Net Income (Loss) Per Common Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Per Common Share	Net Income (Loss) Per Common Share The following table presents the calculation of net income (loss) per common share on a basic and diluted basis: For the Three Months Ended For the Nine Months Ended (in millions, except number of shares and per share amounts) 2021 2020 2021 2020 Net income (loss) $ 22.4 $ (23.1) $ 122.0 $ (53.2) Less: Preferred share dividends 2.6 2.0 7.9 2.0 Net income (loss) attributable to common shareholders 19.8 (25.1) 114.1 (55.2) Weighted average common shares outstanding - basic 34,854,724 34,667,266 34,796,528 34,590,659 Effect of dilutive securities: Equity compensation awards 181,976 — 265,888 — Weighted average common shares outstanding - diluted 35,036,700 34,667,266 35,062,416 34,590,659 Net income (loss) per common share: Basic $ 0.57 $ (0.72) $ 3.28 $ (1.60) Diluted $ 0.56 $ (0.72) $ 3.26 $ (1.60) Excluded from the weighted average common shares outstanding calculation at September 30, 2021 and 2020 are 11,315,889 shares, which are held as treasury shares. The shares are excluded as of their repurchase date. There were 26,422 and 49,796 weighted average shares excluded from the computation of diluted net income per common share because they were anti-dilutive for the three and nine months ended September 30, 2021, respectively. Due to the net loss incurred for the three and nine months ended September 30, 2020, all of the potentially dilutive securities were anti-dilutive, and therefore, omitted from the calculation.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information Interest paid and income taxes paid (recovered) were as follows: For the Nine Months Ended (in millions) 2021 2020 Senior unsecured fixed rate notes $ 7.0 $ 7.0 Junior subordinated debentures 7.5 9.4 Other indebtedness 1.5 4.6 Total interest paid $ 16.0 $ 21.0 Income taxes paid 30.7 46.0 Income taxes recovered (1.2) (1.6) Income taxes paid, net $ 29.5 $ 44.4

Share-based Compensation

Share-based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Share-based Compensation	Share-based Compensation Argo Group’s 2019 Omnibus Incentive Plan In May 2019, our shareholders approved the 2019 Omnibus Incentive Plan (the “2019 Plan”), which provides equity-based and cash-based performance-related incentives to key employees, non-employee directors and other service providers. The intent of the 2019 Plan is to encourage and provide for the acquisition of an ownership interest in Argo Group, enabling us to attract and retain qualified and competent persons to serve as members of our management team and Board of Directors. The 2019 Plan authorizes 1,885,000 common shares to be granted as equity-based awards. No further grants will be made under any prior plan; however, any awards under a prior plan that are outstanding as of the effective date shall remain subject to the terms and conditions of, and be governed by, such prior plan. Awards granted under the 2019 Plan may be in the form of stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance awards, other stock-based awards or other cash-based awards. Awards may be granted either alone, in addition to or in tandem with other awards authorized under the 2019 Plan. Awards that are settled in stock will count as one share for the purposes of reducing the share reserve under the 2019 Plan. Shares issued under this plan may be shares that are authorized and unissued or shares that we have reacquired, including shares purchased on the open market. Stock options and stock appreciation rights are required to have an exercise price that is not less than the fair market value on the date of grant. The term of these awards is not to exceed ten years. Restricted Shares A summary of non-vested restricted share activity as of September 30, 2021 and changes during the nine months then ended is as follows: Shares Weighted-Average Outstanding at January 1, 2021 370,027 $ 44.22 Granted 159,382 $ 54.97 Vested and issued (142,705) $ 45.94 Expired or forfeited (92,039) $ 44.41 Outstanding at September 30, 2021 294,665 $ 49.15 The restricted shares vest over one Performance Shares We have issued to certain key employees non-vested restricted stock awards whose vesting is subject to the achievement of certain performance measures. The non-vested performance share awards vest over three A summary of non-vested performance share activity as of September 30, 2021 and changes during the nine months then ended is as follows: Shares Weighted-Average Outstanding at January 1, 2021 157,847 $ 43.10 Granted 100,291 $ 54.79 Vested and issued (8,987) $ 44.49 Expired or forfeited (45,768) $ 48.36 Outstanding at June 30, 2021 203,383 $ 47.62 Expense recognized under this plan for the performance shares was $0.2 million and $1.8 million for the three and nine months ended September 30, 2021, respectively, as compared to $0.2 million and $0.9 million for the three and nine months ended September 30, 2020, respectively. As of September 30, 2021, there was $5.6 million of total unrecognized compensation cost related to performance share compensation arrangements granted by Argo Group.

Underwriting, Acquisition and I

Underwriting, Acquisition and Insurance Expenses	9 Months Ended
	Sep. 30, 2021
Underwriting, Acquisition and Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, Acquisition and Insurance Expenses Underwriting, acquisition and insurance expenses were as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Commissions $ 88.0 $ 79.4 $ 236.2 $ 201.9 Other underwriting, acquisition and insurance expenses 105.1 99.1 315.7 304.3 193.1 178.5 551.9 506.2 Net deferral of policy acquisition costs (16.0) (16.6) (21.1) (16.4) Total underwriting, acquisition and insurance expenses $ 177.1 $ 161.9 $ 530.8 $ 489.8

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes We are incorporated under the laws of Bermuda and, under current Bermuda law, are not obligated to pay any taxes in Bermuda based upon income or capital gains. We have received an undertaking from the Supervisor of Insurance in Bermuda pursuant to the provisions of the Exempted Undertakings Tax Protection Act, 2011, which exempts us from any Bermuda taxes computed on profits, income or any capital asset, gain or appreciation or any tax in the nature of estate, duty or inheritance tax, at least until the year 2035. We do not consider Argo Group International Holdings, Ltd. to be engaged in a trade or business in the United States or the United Kingdom and, accordingly, do not expect to be subject to direct United States or United Kingdom income taxation. We have subsidiaries based in the United Kingdom that are subject to the tax laws of that country. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Certain of the United Kingdom subsidiaries are deemed to be engaged in business in the United States, and therefore, are subject to United States corporate tax in respect of a proportion of their United States underwriting business only. Relief is available against the United Kingdom tax liabilities in respect of overseas taxes paid that arise from the underwriting business. Our United Kingdom subsidiaries file separate United Kingdom income tax returns. We have subsidiaries based in the United States that are subject to United States tax laws. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Our United States subsidiaries generally file a consolidated United States federal income tax return. We also have operations in Brazil, France, Ireland, Italy, Malta, and Switzerland, which also are subject to income taxes imposed by the jurisdiction in which they operate. We have operations in Barbados and the United Arab Emirates, which are not subject to income tax under the laws of those countries. On June 10, 2021, U.K. tax legislation referred to as Finance Act 2021 received Royal Assent and was enacted. The effects of changes in tax laws and tax rates are recognized in the period of enactment. Accordingly, we recorded the impacts of Finance Act 2021 in our June 30, 2021 consolidated financial statements which primarily includes the remeasurement of our deferred tax assets and liabilities for the increased U.K. federal tax rate from 19% to 25% beginning on April 1, 2023. The remeasurement resulted in an increase of net deferred tax assets of $7.4 million. Our expected income tax provision computed on pre-tax income (loss) at the weighted average tax rate has been calculated as the sum of the pre-tax income (loss) in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. For the three and nine months ended September 30, 2021 and 2020, pre-tax income (loss) attributable to our operations and the corresponding operations’ effective tax rates were as follows: For the Three Months Ended September 30, 2021 2020 (in millions) Pre-Tax Effective Pre-Tax Effective Bermuda $ 7.2 — % $ 5.5 0.1 % United States 21.4 26.8 % 7.2 3.1 % United Kingdom (6.1) (1.0) % (38.1) 9.7 % Barbados — — % — — % Belgium — — % 0.2 22.8 % Brazil 5.9 — % 0.3 — % United Arab Emirates 0.9 — % 0.4 — % Ireland — (1) — % — (1) — % Italy 0.2 — % (0.9) — % Malta (1.3) — % (1.1) — % Switzerland — (1) — % — (1) 1.0 % Pre-tax income (loss) $ 28.2 20.6 % $ (26.5) 12.9 % (1) Pre-tax income (loss) for the respective year was less than $0.1 million. For the Nine Months Ended September 30, 2021 2020 (in millions) Pre-Tax Effective Pre-Tax Effective Bermuda $ 14.5 — % $ (49.1) — % United States 126.7 20.7 % 62.5 21.3 % United Kingdom (14.9) 68.4 % (65.7) 9.9 % Barbados — (1) — % — — % Belgium — — % 0.2 25.3 % Brazil 9.8 — % 3.2 — % United Arab Emirates 1.2 — % 1.8 — % Ireland (0.1) — % (0.1) — % Italy 1.4 — % 0.7 — % Malta (0.4) — % 0.2 — % Switzerland (0.1) — % — (1) — % Pre-tax income (loss) $ 138.1 11.7 % $ (46.3) (14.8) % (1) Pre-tax income (loss) for the respective year was less than $0.1 million. Our effective tax rate may vary significantly from period to period depending on the jurisdiction generating the pre-tax income (loss) and its corresponding statutory tax rate. The geographic distribution of pre-tax income (loss) can fluctuate significantly between periods given the inherit nature of our business. A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Income tax provision at expected rate $ 5.6 $ (6.5) 28.4 2.8 Tax effect of: Nontaxable investment income (0.1) (0.2) (0.4) (0.5) Foreign exchange adjustments 0.2 0.7 (0.3) 0.1 Goodwill — — (1) — 1.0 Withholding taxes — — (1) 0.1 0.1 Prior period adjustments — 0.9 — 0.9 Change in uncertain tax position liability (0.4) — (2.3) — Change in valuation allowance 0.8 1.2 (1.1) 1.2 Impact of change in tax rate related to Finance Act 2021 — — (7.4) — Other (0.3) 0.5 (0.9) 1.3 Income tax provision (benefit) $ 5.8 $ (3.4) $ 16.1 $ 6.9 (1) Tax effect of the adjustment for the respective year was less than $0.1 million. Our gross deferred tax assets are supported by taxes paid in previous periods, reversal of taxable temporary differences and recognition of future taxable income. Management regularly evaluates the recoverability of the deferred tax assets and makes any necessary adjustments to them based upon any changes in management’s expectations of future taxable income. Realization of deferred tax assets is dependent upon our generation of future taxable income sufficient to recover tax benefits that cannot be recovered from taxes paid in the carryback period, generally for our U.S. property and casualty insurers two years for net operating losses and for all our U.S. subsidiaries three years for capital losses. If a company determines that any of its deferred tax assets will not result in future tax benefits, a valuation allowance must be established for the portion of these assets that are not expected to be realized. For the three and nine months ended September 30, 2021, the net change in valuation allowance for deferred tax assets was an increase (decrease) of $0.8 million and $(1.1) million, respectively, relating to the following: Internal Revenue Code Section 382 limited net operating loss carryforwards within the United States, cumulative losses incurred since inception, and valuation allowances acquired through or related to acquisitions. Based upon a review of our available evidence, both positive and negative discussed above, our management concluded that it is more-likely-than-not that the other deferred tax assets will be realized. For any uncertain tax positions not meeting the “more-likely-than-not” recognition threshold, accounting standards require recognition, measurement and disclosure in a company’s financial statements. For the three and nine months ended September 30, 2021, the Company had a net decrease of uncertain tax positions in the amount of $(0.4) million and $(2.3) million, respectively. A net increase (decrease) of interest in the amount of $0.1 million and $(0.3) million has been recorded in the line item “Interest expense” in our Consolidated Statements of (Loss) Income for the three and nine months ended September 30, 2021, respectively. A net increase (decrease) of penalty in the amount of $0.0 million and $(0.4) million has been recorded in the line “Underwriting, acquisition and insurance expenses” in our Consolidated Statements of Income (Loss) for the three and nine months ended September 30, 2021, respectively. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2017. Our United Kingdom subsidiaries are no longer subject to United Kingdom income tax examinations by Her Majesty’s Revenue and Customs for years before 2019. Numerous foreign jurisdictions in which we operate have provided or proposed income-tax relief in response to the COVID-19 pandemic. The Company does not anticipate any of the recent legislative initiatives to have a material impact on its financial statements and will continue to analyze these initiatives in response to the COVID-19 pandemic.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and ContingenciesArgo Group’s subsidiaries are parties to legal actions incidental to their business. Based on the opinion of legal counsel, management believes that the resolution of these matters will not materially affect our financial condition or results of operations.We have contractual commitments to invest up to $63.0 million related to our limited partnership investments at September 30, 2021, as further disclosed in Note 3, “Investments.” These commitments will be funded as required by the partnership agreements which can be called to be fulfilled at any time, not to exceed twelve years.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Segment Information	Segment Information We are primarily engaged in underwriting property and casualty insurance. We have two ongoing reporting segments: U.S. Operations and International Operations. Additionally, we have a Run-off Lines segment for certain products that we no longer underwrite. We consider many factors, including the nature of each segment’s insurance and reinsurance products, production sources, distribution strategies and the regulatory environment, in determining how to aggregate reporting segments. Transactions between segments are reported in the segment that initiated the transaction. In evaluating the operating performance of our segments, we focus on core underwriting and investing results before the consideration of realized gains or losses from investments. Realized investment gains are reported as a component of the Corporate and Other segment, as decisions regarding the acquisition and disposal of securities reside with the corporate investment function and are not under the control of the individual business segments. Identifiable assets by segment are those assets used in the operation of each segment. Revenue and income (loss) before income taxes for each segment were as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Revenue: Earned premiums U.S. Operations $ 323.5 $ 298.7 $ 952.4 $ 902.8 International Operations 163.9 146.7 471.1 410.8 Run-off Lines 0.1 0.1 0.4 0.3 Total earned premiums 487.5 445.5 1,423.9 1,313.9 Net investment income U.S. Operations 29.2 30.1 91.7 56.1 International Operations 12.3 9.9 38.2 19.0 Run-off Lines 0.9 1.5 2.8 2.8 Corporate and Other 3.7 0.5 10.5 1.1 Total net investment income 46.1 42.0 143.2 79.0 Net realized investment gains (losses) (5.3) (5.7) 32.5 (24.0) Total revenue $ 528.3 $ 481.8 $ 1,599.6 $ 1,368.9 For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Income (loss) before income taxes U.S. Operations $ 39.9 $ 20.1 $ 130.1 $ 88.2 International Operations 6.9 (14.7) 14.6 (54.5) Run-off Lines (3.6) (9.5) (4.5) (10.3) Total segment income before taxes 43.2 (4.1) 140.2 23.4 Corporate and Other (11.0) (8.3) (30.2) (32.1) Net realized investment and other gains (losses) (5.3) (5.7) 32.5 (24.0) Foreign currency exchange losses 1.3 (8.4) (4.4) (13.6) Total income (loss) before income taxes $ 28.2 $ (26.5) $ 138.1 $ (46.3) The table below presents earned premiums by geographic location for the three and nine months ended September 30, 2021 and 2020. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that underwrite the business and not by the location of insureds or reinsureds from whom the business was generated. For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 United States $ 322.7 $ 299.1 $ 949.7 $ 900.2 United Kingdom 129.1 90.6 344.9 252.8 Bermuda 10.3 27.3 46.1 73.8 Malta 7.7 13.0 32.7 43.3 All other jurisdictions 17.7 15.5 50.5 43.8 Total earned premiums $ 487.5 $ 445.5 $ 1,423.9 $ 1,313.9 The following table represents identifiable assets: (in millions) September 30, 2021 December 31, 2020 U.S. Operations $ 5,841.6 $ 6,032.2 International Operations 3,900.6 3,899.4 Run-off Lines 270.3 335.9 Corporate and Other 505.6 198.3 Total $ 10,518.1 $ 10,465.8 Included in total assets at September 30, 2021 and December 31, 2020 are $597.5 million and $825.9 million, respectively, in assets associated with trade capital providers.

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event On October 4, 2021, we reached an agreement to sell our Brazilian operations, Argo Seguros Brasil S.A. (“Argo Brazil”), to Spice Private Equity Ltd., an investment company focused on global private equity investments, for a purchase price of 160 million Brazilian Reais (approximately $29.8 million), subject to the terms and conditions set forth in the purchase agreement. Argo Brazil is one of the units within our International Operations reporting segment. The transaction is subject to regulatory and anti-trust approval and is expected to close in the fourth quarter of 2021. As of the date of the financial statements included in this Quarterly Report on Form 10-Q we are unable to reasonably estimate the gain/loss that will result from this transaction.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. and its subsidiaries (“Argo Group,” “Argo,” “we,” “us,” “our” or the “Company”) have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Argo Group is an underwriter of specialty insurance products in the property and casualty market. The preparation of interim financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for expected credit losses; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment, including the allowance for credit losses on fixed maturity securities; valuation of goodwill and intangibles and our deferred tax asset valuation allowance. Actual results could materially differ from those estimates. Certain financial information that is normally included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2020, filed with the Securities and Exchange Commission ("SEC") on March 15, 2021.
Reclassification	Certain reclassifications have been made to financial information presented for prior years to conform to the current year’s presentation.
Fair Value Measurement	Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability, or in the absence of a principal market, the most advantageous market. Market participants are buyers and sellers in the principal (or most advantageous) market that are independent, knowledgeable, able to transact for the asset or liability and willing to transfer the asset or liability. Valuation techniques consistent with the market approach, income approach and/or cost approach are used to measure fair value. The inputs of these valuation techniques are categorized into three levels. • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that can be accessed at the reporting date. We define actively traded as a security that has traded in the past seven days. We receive one quote per instrument for Level 1 inputs. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. We receive one quote per instrument for Level 2 inputs. • Level 3 inputs are unobservable inputs. Unobservable inputs reflect our own judgments about the assumptions market participants would use in pricing the asset or liability based on the best information available in the circumstances. We receive fair value prices from third-party pricing services and our outside investment managers. These prices are determined using observable market information such as dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. We have reviewed the processes used by the third-party providers for pricing the securities and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of September 30, 2021 and December 31, 2020. A description of the valuation techniques we use to measure assets at fair value is as follows: Fixed Maturities (Available-for-Sale) Levels 1 and 2: • United States Treasury securities are typically valued using Level 1 inputs. For these securities, we obtain fair value measurements from third-party pricing services using quoted prices (unadjusted) in active markets at the reporting date. • United States Government agencies, non-U.S. Government securities, obligations of states and political subdivisions, credit securities and foreign denominated government and credit securities are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, yield curves, live trading levels, trade execution data, credit information and the security’s terms and conditions, among other things. • Asset and mortgage-backed securities and collateralized loan obligations are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. Fixed Maturities (Available-for-Sale) Levels 3: We own term loans that are valued using unobservable inputs. Equity Securities Level 1: Equity securities are principally reported at fair value using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Equity Securities Level 3: We own certain equity securities that are reported at fair value using Level 3 inputs. The valuation techniques for these securities include the following: • Fair value measurements for an investment in an equity fund obtained by applying final prices provided by the administrator of the fund, which is based upon certain estimates and assumptions. • Fair value measurements from brokers and independent valuation services, both based upon estimates, assumptions and other unobservable inputs. Other Investments Level 2: Foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. These assets are invested in short-term government securities, agency securities and corporate bonds and are valued using Level 2 inputs based upon values obtained from Lloyd’s. Short-term Investments: Short-term investments are principally reported at fair value using Level 1 inputs, with the exception of short-term corporate and governmental bonds reported at fair value using Level 2 inputs as described in the fixed maturities section above. Values for the investments categorized as Level 1 are obtained from various financial institutions as of the reporting date.
Allowance for Credit Losses	Allowance for Credit Losses For fixed maturities with a decline in the fair value between the amortized cost due to credit-related factors, an allowance is established for the difference between the estimated recoverable value and amortized cost with a corresponding charge to realized investment losses in the Statement of Income (Loss). The allowance is limited to the difference between amortized cost and fair value. The estimated recoverable value is the present value of cash flows expected to be collected, as determined by management. The difference between fair value and amortized cost that is not associated with credit-related factors is recognized in the Statement of Comprehensive Income (Loss). Accrued interest is excluded from the measurement of the allowance for credit losses. When determining if a credit loss has been incurred, we may consider the historical performance of the security, available market information and security specific considerations such as the priority payment of the security. In addition, inputs used in our analysis include, but are not limited to, credit ratings and downgrades, delinquency rates, missed scheduled interest or principal payments, purchase yields, underlying asset performance, collateral types, modeled default rates, modeled severity rates, call/prepayment rates, expected cash flows, industry concentrations, and potential or filed bankruptcies or restructurings. We evaluate for credit losses each quarter. If we determine that all or a portion of a fixed maturity is uncollectible, the uncollectible amortized cost is written off with a corresponding reduction to the allowance for credit losses. If we collect cash flows that were previously written off, the recovery is recognized in realized investment gains. We also consider whether we intend to sell an available-for-sale security or if it is more likely than not that we will be required to sell the security before recovery of its amortized cost. In these instances, a decline in fair value is recognized in net realized gains (losses) in the Statement of Income based on the fair value of the security at the time of assessment, resulting in a new cost basis for the security.
Disclosures about Fair Value of Financial Instruments	Disclosures about Fair Value of Financial Instruments Cash. The carrying amount approximates fair value. Investment securities and short-term investments. See Note 3, “Investments,” for additional information. Premiums receivable and reinsurance recoverables on paid losses. The carrying value of current receivables and reinsurance recoverables on paid losses approximates fair value. Debt. At September 30, 2021 and December 31, 2020, the fair value of our debt instruments is determined using both Level 1 and Level 2 inputs, as previously defined in Note 3, “Investments.” We receive fair value prices from third-party pricing services for our financial instruments as well as for similar financial instruments. These prices are determined using observable market information such as publicly traded quoted prices, and trading prices for similar financial instruments actively being traded in the current market. We have reviewed the processes used by the third-party providers for pricing the securities and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of September 30, 2021 and December 31, 2020. A description of the valuation techniques we use to measure these liabilities at fair value is as follows: Senior Unsecured Fixed Rate Notes Level 1: • Our senior unsecured fixed rate notes are valued using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Junior Subordinated Debentures and Floating Rate Loan Stock Level 2: • Our trust preferred debentures, subordinated debentures and floating rate loan stock are typically valued using Level 2 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices for similar securities being traded in active markets at the reporting date, as our specific debt instruments are more infrequently traded.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments	The amortized cost, gross unrealized gains, gross unrealized losses, allowance for credit losses, and fair value of fixed maturity investments were as follows: September 30, 2021 (in millions) Amortized Gross Gross Allowance for Credit Losses Fair Fixed maturities U.S. Governments $ 404.8 $ 7.5 $ 2.1 $ — $ 410.2 Foreign Governments 229.3 2.7 2.7 0.2 229.1 Obligations of states and political subdivisions 169.9 6.7 0.9 — 175.7 Corporate bonds 1,996.2 47.8 14.4 6.6 2,023.0 Commercial mortgage-backed securities 403.1 9.1 3.0 — 409.2 Residential mortgage-backed securities 488.1 10.6 4.8 — 493.9 Asset-backed securities 110.9 2.2 0.1 0.2 112.8 Collateralized loan obligations 342.9 1.8 1.3 — 343.4 Total fixed maturities $ 4,145.2 $ 88.4 $ 29.3 $ 7.0 $ 4,197.3 December 31, 2020 (in millions) Amortized Gross Gross Allowance for Credit Losses Fair Fixed maturities U.S. Governments $ 385.4 $ 14.7 $ 0.3 $ — $ 399.8 Foreign Governments 284.1 11.6 0.7 0.2 294.8 Obligations of states and political subdivisions 163.1 7.7 0.3 0.1 170.4 Corporate bonds 1,925.9 75.3 13.3 6.1 1,981.8 Commercial mortgage-backed securities 324.8 15.2 0.3 — 339.7 Residential mortgage-backed securities 491.4 17.4 0.6 — 508.2 Asset-backed securities 120.5 2.9 0.4 0.2 122.8 Collateralized loan obligations 285.9 4.9 1.2 — 289.6 Total fixed maturities $ 3,981.1 $ 149.7 $ 17.1 $ 6.6 $ 4,107.1
Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity	The amortized cost and fair values of fixed maturity investments as of September 30, 2021, by contractual maturity, were as follows: (in millions) Amortized Fair Due in one year or less $ 288.7 $ 292.9 Due after one year through five years 1,670.6 1,695.1 Due after five years through ten years 719.6 728.6 Due after ten years 121.3 121.4 Structured securities 1,345.0 1,359.3 Total $ 4,145.2 $ 4,197.3
Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio	Details regarding the carrying value and unfunded investment commitments of other investments as of September 30, 2021 and December 31, 2020 were as follows: September 30, 2021 (in millions) Carrying Unfunded Investment Type Hedge funds $ 102.1 $ — Private equity 254.3 63.0 Overseas deposits 64.6 — Other 4.8 — Total other investments $ 425.8 $ 63.0 December 31, 2020 (in millions) Carrying Unfunded Investment Type Hedge funds $ 111.2 $ — Private equity 211.4 80.0 Overseas deposits 102.1 — Other 4.7 — Total other investments $ 429.4 $ 80.0
Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments	An aging of unrealized losses on our investments in fixed maturities is presented below: September 30, 2021 Less Than One Year One Year or Greater Total (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Fixed maturities U.S. Governments $ 161.1 $ 1.7 $ 5.6 $ 0.4 $ 166.7 $ 2.1 Foreign Governments 117.7 2.4 1.3 0.3 119.0 2.7 Obligations of states and political subdivisions 28.3 0.5 0.4 0.4 28.7 0.9 Corporate bonds 626.4 10.8 40.7 3.6 667.1 14.4 Commercial mortgage-backed securities 136.9 2.6 8.9 0.4 145.8 3.0 Residential mortgage-backed securities 207.4 4.6 4.8 0.2 212.2 4.8 Asset-backed securities 21.3 0.1 — — 21.3 0.1 Collateralized loan obligations 126.8 1.3 2.4 — 129.2 1.3 Total fixed maturities $ 1,425.9 $ 24.0 $ 64.1 $ 5.3 $ 1,490.0 $ 29.3 December 31, 2020 Less Than One Year One Year or Greater Total (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Fixed maturities U.S. Governments $ 40.6 $ 0.3 $ — $ — $ 40.6 $ 0.3 Foreign Governments 18.0 0.5 0.1 0.2 18.1 0.7 Obligations of states and political subdivisions 5.2 0.3 — — 5.2 0.3 Corporate bonds 202.5 6.7 17.5 6.6 220.0 13.3 Commercial mortgage-backed securities 21.8 0.3 — — 21.8 0.3 Residential mortgage-backed securities 74.4 0.4 3.0 0.2 77.4 0.6 Asset-backed securities 4.6 0.4 — — 4.6 0.4 Collateralized loan obligations 121.1 0.9 49.1 0.3 170.2 1.2 Total fixed maturities $ 488.2 $ 9.8 $ 69.7 $ 7.3 $ 557.9 $ 17.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	The following table presents a roll-forward of the changes in allowance for credit losses on available-for-sale fixed maturities by industry category for the three and nine months ending September 30, 2021 and 2020, respectively: (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, June 30, 2021 $ 0.2 $ — $ 6.2 $ — $ 6.4 Securities for which allowance was not previously recorded — — 0.7 0.2 0.9 Securities sold during the period — — (0.2) — (0.2) Additional net increases (decreases) in existing allowance — — (0.1) — (0.1) Ending balance, September 30, 2021 $ 0.2 $ — $ 6.6 $ 0.2 $ 7.0 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, June 30, 2020 $ 0.3 $ 0.1 $ 39.7 $ 0.1 $ 40.2 Securities for which allowance was not previously recorded 0.1 — 13.6 — 13.7 Securities sold during the period (0.2) — (14.7) — (14.9) Additional net increases (decreases) in existing allowance — 0.1 (6.4) 1.0 (5.3) Ending balance, September 30, 2020 $ 0.2 $ 0.2 $ 32.2 $ 1.1 $ 33.7 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, January 1, 2021 $ 0.2 $ 0.1 $ 6.1 $ 0.2 $ 6.6 Securities for which allowance was not previously recorded — — 2.7 0.2 2.9 Securities sold during the period — — (1.1) — (1.1) Additional net increases (decreases) in existing allowance — (0.1) (1.1) (0.2) (1.4) Ending balance, September 30, 2021 $ 0.2 $ — $ 6.6 $ 0.2 $ 7.0 (in millions) Foreign Governments Obligations of states and political subdivisions Corporate bonds Asset backed securities Total Beginning balance, January 1, 2020 $ — $ — $ — $ — $ — Additions-initial adoption of accounting standard — — 6.8 0.1 6.9 Securities for which allowance was not previously recorded 0.3 0.3 14.4 — 15.0 Securities sold during the period (0.2) — (15.5) — (15.7) Additional net increases (decreases) in existing allowance 0.1 (0.1) 26.5 1.0 27.5 Ending balance, September 30, 2020 $ 0.2 $ 0.2 $ 32.2 $ 1.1 $ 33.7
Schedule of Company's Gross Realized Investment Gains (Losses)	The following table presents our gross realized investment gains (losses): For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Realized gains on fixed maturities and other Fixed maturities $ 3.3 $ 7.2 $ 26.3 $ 30.5 Other investments, including short-terms 0.9 10.8 8.9 70.7 Other assets — — 3.4 32.3 4.2 18.0 38.6 133.5 Realized losses on fixed maturities and other Fixed maturities (0.4) (7.5) (5.0) (25.7) Other investments, including short-terms (3.3) (12.6) (16.6) (58.4) Other assets (1.4) (0.4) (14.0) (0.9) Credit losses on fixed maturities (0.7) (10.5) (1.5) (43.0) (5.8) (31.0) (37.1) (128.0) Equity securities Net realized gains (losses) on equity securities 1.4 (3.2) 0.3 (15.8) Change in unrealized gains (losses) on equity securities held at the end of the period (5.1) 10.5 30.7 (13.7) Net realized gains (losses) on equity securities (3.7) 7.3 31.0 (29.5) Net realized investment and other gains (losses) before income taxes (5.3) (5.7) 32.5 (24.0) Income tax (benefit) provision — (0.5) — (1.7) Net realized investment gains (losses), net of income taxes $ (5.3) $ (5.2) $ 32.5 $ (22.3)
Schedule of Changes in Unrealized Appreciation (Depreciation)	Changes in unrealized gains (losses) related to investments are summarized as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Change in unrealized gains (losses) Fixed maturities $ (23.0) $ 42.5 $ (70.7) $ 66.9 Other investments — 0.2 — (14.0) Other and short-term investments (0.3) — (0.6) — Net unrealized investment gains (losses) before income taxes (23.3) 42.7 (71.3) 52.9 Income tax provision (benefit) (4.4) 7.0 (13.0) 12.2 Net unrealized investment gains (losses), net of income taxes $ (18.9) $ 35.7 $ (58.3) $ 40.7
Schedule of Fair Value of Foreign Currency Exchange Forward Contracts	The fair value of our foreign currency exchange forward contracts as of September 30, 2021 and December 31, 2020 was as follows: (in millions) September 30, 2021 December 31, 2020 Operational currency exposure $ (0.3) $ 0.4 Asset manager investment exposure 2.0 (0.2) Total return strategy (3.4) 0.7 Total $ (1.7) $ 0.9
Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts	The following table represents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Realized gains Operational currency exposure $ — $ 4.2 $ 7.1 $ 10.1 Asset manager investment exposure 1.0 — 2.8 1.0 Total return strategy 6.9 10.3 13.0 43.5 Gross realized investment gains 7.9 14.5 22.9 54.6 Realized losses Operational currency exposure (5.2) (1.8) (17.8) (6.5) Asset manager investment exposure — (0.9) (0.8) (1.9) Total return strategy (6.9) (10.4) (12.0) (47.0) Gross realized investment losses (12.1) (13.1) (30.6) (55.4) Net realized investment (losses) gains on foreign currency exchange forward contracts $ (4.2) $ 1.4 $ (7.7) $ (0.8)
Schedule of Restricted Assets	The following table presents our components of restricted assets: (in millions) September 30, 2021 December 31, 2020 Securities on deposit for regulatory and other purposes $ 205.3 $ 227.5 Securities pledged as collateral for letters of credit and other 191.0 189.4 Securities and cash on deposit supporting Lloyd’s business (1) 331.2 409.2 Total restricted investments $ 727.5 $ 826.1 (1) As the corporate member for our Lloyd’s syndicates, Argo’s share of Argo (No. 604) Limited’s required Funds to maintain at Lloyd’s was $397.9 million at September 30, 2021, of which $179.0 million was provided by Argo Re, Ltd. (“Argo Re.”)
Financial Assets Measured at Fair Value on Recurring Basis	Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair Value Measurements at Reporting Date Using (in millions) September 30, Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 410.2 $ 398.3 $ 11.9 $ — Foreign Governments 229.1 — 229.1 — Obligations of states and political subdivisions 175.7 — 175.7 — Corporate bonds 2,023.0 — 2,019.2 3.8 Commercial mortgage-backed securities 409.2 — 409.2 — Residential mortgage-backed securities 493.9 — 493.9 — Asset-backed securities 112.8 — 112.8 — Collateralized loan obligations 343.4 — 343.4 — Total fixed maturities 4,197.3 398.3 3,795.2 3.8 Equity securities 181.8 156.3 — 25.5 Other investments 65.1 — 65.1 — Short-term investments 508.6 497.4 11.2 — $ 4,952.8 $ 1,052.0 $ 3,871.5 $ 29.3 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 399.8 $ 383.5 $ 16.3 $ — Foreign Governments 294.8 — 294.8 — Obligations of states and political subdivisions 170.4 — 170.4 — Corporate bonds 1,981.8 — 1,974.8 7.0 Commercial mortgage-backed securities 339.7 — 339.7 — Residential mortgage-backed securities 508.2 — 508.2 — Asset-backed securities 122.8 — 122.8 — Collateralized loan obligations 289.6 — 289.6 — Total fixed maturities 4,107.1 383.5 3,716.6 7.0 Equity securities 176.7 159.2 — 17.5 Other investments 102.5 0.4 102.1 — Short-term investments 542.6 526.5 16.1 — $ 4,928.9 $ 1,069.6 $ 3,834.8 $ 24.5 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs
Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3	A reconciliation of the beginning and ending balances for the investments categorized as Level 3 are as follows: Fair Value Measurements Using Observable Inputs (Level 3) (in millions) Corporate Bonds Equity Short Term Investments Total Beginning balance, January 1, 2021 $ 7.0 $ 17.5 $ — $ 24.5 Transfers into Level 3 — 1.5 — 1.5 Transfers out of Level 3 — — — — Total gains or losses (realized/unrealized): Included in net income 0.1 6.5 — 6.6 Included in other comprehensive income 0.1 — — 0.1 Purchases, issuances, sales, and settlements: Purchases 0.1 — — 0.1 Issuances — — — — Sales (3.5) — — (3.5) Settlements — — — — Ending balance, September 30, 2021 $ 3.8 $ 25.5 $ — $ 29.3 Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at September 30, 2021 $ — $ — $ — $ — (in millions) Credit Financial Equity Total Beginning balance, January 1, 2020 $ 7.4 $ 18.2 $ 25.6 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income — (5.9) (5.9) Included in other comprehensive loss (0.5) — (0.5) Purchases, issuances, sales, and settlements: Purchases 0.1 5.2 5.3 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2020 $ 7.0 $ 17.5 $ 24.5 Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2020 $ — $ — $ —

Allowance for Credit Losses (Ta

Allowance for Credit Losses (Tables)	9 Months Ended
	Sep. 30, 2021
Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss	The following table represents the balances of premiums receivable, net of allowance for expected credit losses, at September 30, 2021 and January 1, 2021, and the changes in the allowance for expected credit losses for the nine months ended September 30, 2021. (in millions) Premiums Receivable, Net of Allowance for Estimated Uncollectible Premiums Allowance for Estimated Uncollectible Premiums Balance, January 1, 2021 $ 679.8 $ 9.4 Current period change for estimated uncollectible premiums — Write-offs of uncollectible premiums receivable (0.6) Foreign exchange adjustments — Balance, September 30, 2021 $ 701.9 $ 8.8
Reinsurance Recoverable, Allowance for Credit Loss	Reinsurance Recoverables The following table presents the balances of reinsurance recoverables, net of the allowance for estimated uncollectible reinsurance, at September 30, 2021 and January 1, 2021, and changes in the allowance for estimated uncollectible reinsurance for the nine months ended September 30, 2021. (in millions) Reinsurance Recoverables, Net of Allowance for Estimated Uncollectible Reinsurance Allowance for Estimated Uncollectible Reinsurance Balance, January 1, 2021 $ 3,009.0 $ 4.1 Current period change for estimated uncollectible reinsurance (0.3) Write-offs of uncollectible reinsurance recoverables — Balance, September 30, 2021 $ 2,954.0 $ 3.8

Reserves for Losses and Loss _2

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Nine Months Ended (in millions) 2021 2020 Net reserves beginning of the year $ 2,906.1 $ 2,722.7 Net AIL reserves acquired — 27.9 Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 884.9 876.9 Prior accident years 6.0 6.1 Losses and LAE incurred during calendar year, net of reinsurance 890.9 883.0 Deduct: Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior (1) 219.7 — Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 100.5 188.2 Prior accident years 552.8 632.7 Losses and LAE payments made during current calendar year, net of reinsurance: 653.3 820.9 Change in participation interest (2) 12.5 33.0 Foreign exchange adjustments (7.3) 23.0 Net reserves - end of period 2,929.2 2,868.7 Add: Reinsurance recoverables on unpaid losses and LAE, end of period 2,510.3 2,521.3 Gross reserves - end of period $ 5,439.5 $ 5,390.0 (1) Amount represents reserves ceded under the reinsurance to close transaction with RiverStone for Lloyd’s years of account 2017 and prior, effective January 1, 2020. (2) Amount represents the change in reserves due to changing our participation in Syndicates 1200 and 1910.
Impact from (Favorable) Unfavorable Development of Prior Accident Years’ Loss and LAE Reserves on Each Reporting Segment	The impact from the (favorable) unfavorable development of prior accident years’ loss and LAE reserves on each reporting segment is presented below: For the Nine Months Ended (in millions) 2021 2020 U.S. Operations $ (0.7) $ (0.5) International Operations 0.1 (4.5) Run-off Lines 6.6 11.1 Total (favorable) unfavorable prior-year development $ 6.0 $ 6.1

Disclosures about Fair Value _2

Disclosures about Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value	A summary of our financial instruments whose carrying value did not equal fair value is shown below: September 30, 2021 December 31, 2020 (in millions) Carrying Fair Carrying Fair Junior subordinated debentures: Trust preferred debentures $ 172.7 $ 173.1 $ 172.7 $ 173.6 Subordinated debentures 85.4 92.0 85.1 92.3 Total junior subordinated debentures 258.1 265.1 257.8 265.9 Senior unsecured fixed rate notes 140.3 150.1 140.2 146.7 Floating rate loan stock 58.7 58.9 60.7 61.0 457.1 474.1 458.7 473.6
Fair Value, Assets Measured on Recurring and Nonrecurring Basis	Based on an analysis of the inputs, our financial instruments measured at fair value for disclosure purposes have been categorized as follows: Fair Value Measurements at Reporting Date Using (in millions) September 30, 2021 Level 1 (a) Level 2 (b) Level 3 (c) Junior subordinated debentures: Trust preferred debentures $ 173.1 $ — $ 173.1 $ — Subordinated debentures 92.0 — 92.0 — Total junior subordinated debentures 265.1 — 265.1 — Senior unsecured fixed rate notes 150.1 150.1 — — Floating rate loan stock 58.9 — 58.9 — 474.1 150.1 324.0 — (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2020 Level 1 (a) Level 2 (b) Level 3 (c) Junior subordinated debentures: Trust preferred debentures $ 173.6 $ — $ 173.6 $ — Subordinated debentures 92.3 — 92.3 — Total junior subordinated debentures 265.9 — 265.9 — Senior unsecured fixed rate notes 146.7 146.7 — — Floating rate loan stock 61.0 — 61.0 — 473.6 146.7 326.9 — (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Changes in Accumulated Other Comprehensive (Loss) Income	A summary of changes in accumulated other comprehensive (loss) income, net of taxes (where applicable) by component for the nine months ended September 30, 2021 and 2020 is presented below: (in millions) Foreign Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2021 $ (37.9) $ 105.1 $ (8.6) $ 58.6 Other comprehensive income (loss) before reclassifications 0.2 (49.2) 1.5 (47.5) Amounts reclassified from accumulated other comprehensive loss — (9.1) — (9.1) Net current-period other comprehensive income (loss) 0.2 (58.3) 1.5 (56.6) Balance, September 30, 2021 $ (37.7) $ 46.8 $ (7.1) $ 2.0 (in millions) Foreign Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2020 $ (22.6) $ 33.5 $ (8.1) $ 2.8 Other comprehensive income (loss) before reclassifications (12.3) 55.9 — 43.6 Amounts reclassified from accumulated other comprehensive loss — (15.2) — (15.2) Net current-period other comprehensive income (loss) (12.3) 40.7 — 28.4 Cumulative effect of adoption of ASU 2016-13 — 5.7 — 5.7 Balance, September 30, 2020 $ (34.9) $ 79.9 $ (8.1) $ 36.9
Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income	The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income (Loss): For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Unrealized gains and losses on securities: Net realized investment losses (gains) $ 8.2 $ 4.5 $ (11.0) $ (15.4) (Benefit) provision for income taxes (0.4) (0.5) 1.9 0.2 Net of taxes $ 7.8 $ 4.0 $ (9.1) $ (15.2)

Net Income (Loss) Per Common _2

Net Income (Loss) Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Per Common Share on Basic and Diluted Basis	The following table presents the calculation of net income (loss) per common share on a basic and diluted basis: For the Three Months Ended For the Nine Months Ended (in millions, except number of shares and per share amounts) 2021 2020 2021 2020 Net income (loss) $ 22.4 $ (23.1) $ 122.0 $ (53.2) Less: Preferred share dividends 2.6 2.0 7.9 2.0 Net income (loss) attributable to common shareholders 19.8 (25.1) 114.1 (55.2) Weighted average common shares outstanding - basic 34,854,724 34,667,266 34,796,528 34,590,659 Effect of dilutive securities: Equity compensation awards 181,976 — 265,888 — Weighted average common shares outstanding - diluted 35,036,700 34,667,266 35,062,416 34,590,659 Net income (loss) per common share: Basic $ 0.57 $ (0.72) $ 3.28 $ (1.60) Diluted $ 0.56 $ (0.72) $ 3.26 $ (1.60)

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Schedule of Interest Paid and Income Taxes Paid (Recovered)	Interest paid and income taxes paid (recovered) were as follows: For the Nine Months Ended (in millions) 2021 2020 Senior unsecured fixed rate notes $ 7.0 $ 7.0 Junior subordinated debentures 7.5 9.4 Other indebtedness 1.5 4.6 Total interest paid $ 16.0 $ 21.0 Income taxes paid 30.7 46.0 Income taxes recovered (1.2) (1.6) Income taxes paid, net $ 29.5 $ 44.4

Share-based Compensation (Table

Share-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Summary of Restricted Share Activity	A summary of non-vested restricted share activity as of September 30, 2021 and changes during the nine months then ended is as follows: Shares Weighted-Average Outstanding at January 1, 2021 370,027 $ 44.22 Granted 159,382 $ 54.97 Vested and issued (142,705) $ 45.94 Expired or forfeited (92,039) $ 44.41 Outstanding at September 30, 2021 294,665 $ 49.15
Schedule of Performance Shares	A summary of non-vested performance share activity as of September 30, 2021 and changes during the nine months then ended is as follows: Shares Weighted-Average Outstanding at January 1, 2021 157,847 $ 43.10 Granted 100,291 $ 54.79 Vested and issued (8,987) $ 44.49 Expired or forfeited (45,768) $ 48.36 Outstanding at June 30, 2021 203,383 $ 47.62

Underwriting, Acquisition and_2

Underwriting, Acquisition and Insurance Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Underwriting, Acquisition and Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, acquisition and insurance expenses were as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Commissions $ 88.0 $ 79.4 $ 236.2 $ 201.9 Other underwriting, acquisition and insurance expenses 105.1 99.1 315.7 304.3 193.1 178.5 551.9 506.2 Net deferral of policy acquisition costs (16.0) (16.6) (21.1) (16.4) Total underwriting, acquisition and insurance expenses $ 177.1 $ 161.9 $ 530.8 $ 489.8

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates	For the three and nine months ended September 30, 2021 and 2020, pre-tax income (loss) attributable to our operations and the corresponding operations’ effective tax rates were as follows: For the Three Months Ended September 30, 2021 2020 (in millions) Pre-Tax Effective Pre-Tax Effective Bermuda $ 7.2 — % $ 5.5 0.1 % United States 21.4 26.8 % 7.2 3.1 % United Kingdom (6.1) (1.0) % (38.1) 9.7 % Barbados — — % — — % Belgium — — % 0.2 22.8 % Brazil 5.9 — % 0.3 — % United Arab Emirates 0.9 — % 0.4 — % Ireland — (1) — % — (1) — % Italy 0.2 — % (0.9) — % Malta (1.3) — % (1.1) — % Switzerland — (1) — % — (1) 1.0 % Pre-tax income (loss) $ 28.2 20.6 % $ (26.5) 12.9 % (1) Pre-tax income (loss) for the respective year was less than $0.1 million. For the Nine Months Ended September 30, 2021 2020 (in millions) Pre-Tax Effective Pre-Tax Effective Bermuda $ 14.5 — % $ (49.1) — % United States 126.7 20.7 % 62.5 21.3 % United Kingdom (14.9) 68.4 % (65.7) 9.9 % Barbados — (1) — % — — % Belgium — — % 0.2 25.3 % Brazil 9.8 — % 3.2 — % United Arab Emirates 1.2 — % 1.8 — % Ireland (0.1) — % (0.1) — % Italy 1.4 — % 0.7 — % Malta (0.4) — % 0.2 — % Switzerland (0.1) — % — (1) — % Pre-tax income (loss) $ 138.1 11.7 % $ (46.3) (14.8) % (1) Pre-tax income (loss) for the respective year was less than $0.1 million.
Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate	A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Income tax provision at expected rate $ 5.6 $ (6.5) 28.4 2.8 Tax effect of: Nontaxable investment income (0.1) (0.2) (0.4) (0.5) Foreign exchange adjustments 0.2 0.7 (0.3) 0.1 Goodwill — — (1) — 1.0 Withholding taxes — — (1) 0.1 0.1 Prior period adjustments — 0.9 — 0.9 Change in uncertain tax position liability (0.4) — (2.3) — Change in valuation allowance 0.8 1.2 (1.1) 1.2 Impact of change in tax rate related to Finance Act 2021 — — (7.4) — Other (0.3) 0.5 (0.9) 1.3 Income tax provision (benefit) $ 5.8 $ (3.4) $ 16.1 $ 6.9 (1) Tax effect of the adjustment for the respective year was less than $0.1 million.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Revenue and Income (Loss) Before Income Taxes for Each Segment	Revenue and income (loss) before income taxes for each segment were as follows: For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Revenue: Earned premiums U.S. Operations $ 323.5 $ 298.7 $ 952.4 $ 902.8 International Operations 163.9 146.7 471.1 410.8 Run-off Lines 0.1 0.1 0.4 0.3 Total earned premiums 487.5 445.5 1,423.9 1,313.9 Net investment income U.S. Operations 29.2 30.1 91.7 56.1 International Operations 12.3 9.9 38.2 19.0 Run-off Lines 0.9 1.5 2.8 2.8 Corporate and Other 3.7 0.5 10.5 1.1 Total net investment income 46.1 42.0 143.2 79.0 Net realized investment gains (losses) (5.3) (5.7) 32.5 (24.0) Total revenue $ 528.3 $ 481.8 $ 1,599.6 $ 1,368.9 For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 Income (loss) before income taxes U.S. Operations $ 39.9 $ 20.1 $ 130.1 $ 88.2 International Operations 6.9 (14.7) 14.6 (54.5) Run-off Lines (3.6) (9.5) (4.5) (10.3) Total segment income before taxes 43.2 (4.1) 140.2 23.4 Corporate and Other (11.0) (8.3) (30.2) (32.1) Net realized investment and other gains (losses) (5.3) (5.7) 32.5 (24.0) Foreign currency exchange losses 1.3 (8.4) (4.4) (13.6) Total income (loss) before income taxes $ 28.2 $ (26.5) $ 138.1 $ (46.3)
Schedule of Earned Premiums by Geographic Location	The table below presents earned premiums by geographic location for the three and nine months ended September 30, 2021 and 2020. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that underwrite the business and not by the location of insureds or reinsureds from whom the business was generated. For the Three Months Ended For the Nine Months Ended (in millions) 2021 2020 2021 2020 United States $ 322.7 $ 299.1 $ 949.7 $ 900.2 United Kingdom 129.1 90.6 344.9 252.8 Bermuda 10.3 27.3 46.1 73.8 Malta 7.7 13.0 32.7 43.3 All other jurisdictions 17.7 15.5 50.5 43.8 Total earned premiums $ 487.5 $ 445.5 $ 1,423.9 $ 1,313.9
Identifiable Assets	The following table represents identifiable assets: (in millions) September 30, 2021 December 31, 2020 U.S. Operations $ 5,841.6 $ 6,032.2 International Operations 3,900.6 3,899.4 Run-off Lines 270.3 335.9 Corporate and Other 505.6 198.3 Total $ 10,518.1 $ 10,465.8

Basis of Presentation - Sale of

Basis of Presentation - Sale of Business Unit (Details) - Disposal Group, Disposed of by Sale, Not Discontinued Operations - Trident $ in Millions	3 Months Ended
	Jun. 30, 2021USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposal, reduction in the original gain on sale	$ (16.5)
Gain on disposal	$ 5

Investments - Additional Inform

Investments - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)security	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)security	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Investments, Debt and Equity Securities [Abstract]
Fair value of investments, assets managed on behalf of the trade capital providers	$ 97.1		$ 97.1		$ 140.3
Number of securities in an unrealized loss position, total | security	5,377		5,377
Number of securities in an unrealized loss position for less than one year | security	577		577
Number of securities in an unrealized loss position for a period of one year or greater | security	85		85
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Fair value of investments, assets managed on behalf of the trade capital providers	$ 97.1		$ 97.1		$ 140.3
Credit losses on fixed maturities	0.8	$ 10.5	1.5	$ 43
Gain (Loss) on Investments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Credit losses on fixed maturities	$ 0.8	$ 10.5	$ 1.5	$ 43

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Fixed maturities
Amortized Cost	$ 4,145.2		$ 3,981.1
Allowance for Credit Losses	7	$ 6.4	6.6	$ 33.7	$ 40.2	$ 0
Fair Value	4,197.3		4,107.1
Fixed maturities
Fixed maturities
Amortized Cost	4,145.2		3,981.1
Gross Unrealized Gains	88.4		149.7
Gross Unrealized Losses	29.3		17.1
Allowance for Credit Losses	7		6.6
Fair Value	4,197.3		4,107.1
Fixed maturities | U.S. Governments
Fixed maturities
Amortized Cost	404.8		385.4
Gross Unrealized Gains	7.5		14.7
Gross Unrealized Losses	2.1		0.3
Allowance for Credit Losses	0		0
Fair Value	410.2		399.8
Fixed maturities | Foreign Governments
Fixed maturities
Amortized Cost	229.3		284.1
Gross Unrealized Gains	2.7		11.6
Gross Unrealized Losses	2.7		0.7
Allowance for Credit Losses	0.2		0.2
Fair Value	229.1		294.8
Fixed maturities | Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	169.9		163.1
Gross Unrealized Gains	6.7		7.7
Gross Unrealized Losses	0.9		0.3
Allowance for Credit Losses	0		0.1
Fair Value	175.7		170.4
Fixed maturities | Corporate bonds
Fixed maturities
Amortized Cost	1,996.2		1,925.9
Gross Unrealized Gains	47.8		75.3
Gross Unrealized Losses	14.4		13.3
Allowance for Credit Losses	6.6		6.1
Fair Value	2,023		1,981.8
Fixed maturities | Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	403.1		324.8
Gross Unrealized Gains	9.1		15.2
Gross Unrealized Losses	3		0.3
Allowance for Credit Losses	0		0
Fair Value	409.2		339.7
Fixed maturities | Residential mortgage-backed securities
Fixed maturities
Amortized Cost	488.1		491.4
Gross Unrealized Gains	10.6		17.4
Gross Unrealized Losses	4.8		0.6
Allowance for Credit Losses	0		0
Fair Value	493.9		508.2
Fixed maturities | Asset-backed securities
Fixed maturities
Amortized Cost	110.9		120.5
Gross Unrealized Gains	2.2		2.9
Gross Unrealized Losses	0.1		0.4
Allowance for Credit Losses	0.2		0.2
Fair Value	112.8		122.8
Fixed maturities | Collateralized loan obligations
Fixed maturities
Amortized Cost	342.9		285.9
Gross Unrealized Gains	1.8		4.9
Gross Unrealized Losses	1.3		1.2
Allowance for Credit Losses	0		0
Fair Value	$ 343.4		$ 289.6

Investments - Schedule of Amo_2

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2021USD ($)
Amortized Cost
Due in one year or less	$ 288.7
Due after one year through five years	1,670.6
Due after five years through ten years	719.6
Due after ten years	121.3
Structured securities	1,345
Total	4,145.2
Fair Value
Due in one year or less	292.9
Due after one year through five years	1,695.1
Due after five years through ten years	728.6
Due after ten years	121.4
Structured securities	1,359.3
Total	$ 4,197.3

Investments - Schedule of Carry

Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Carrying Value	$ 5,313.5	$ 5,255.8
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	102.1	111.2
Unfunded Commitments	0	0
Private equity
Schedule of Investments [Line Items]
Carrying Value	254.3	211.4
Unfunded Commitments	63	80
Overseas deposits
Schedule of Investments [Line Items]
Carrying Value	64.6	102.1
Unfunded Commitments	0	0
Other
Schedule of Investments [Line Items]
Carrying Value	4.8	4.7
Unfunded Commitments	0	0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	425.8	429.4
Unfunded Commitments	$ 63	$ 80

Investments - Schedule of Aging

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - Fixed maturities - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 1,425.9	$ 488.2
Unrealized Losses, Less Than One Year	24	9.8
Fair Value, One Year or Greater	64.1	69.7
Unrealized Losses, One Year or Greater	5.3	7.3
Fair Value, Total	1,490	557.9
Unrealized Losses, Total	29.3	17.1
U.S. Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	161.1	40.6
Unrealized Losses, Less Than One Year	1.7	0.3
Fair Value, One Year or Greater	5.6	0
Unrealized Losses, One Year or Greater	0.4	0
Fair Value, Total	166.7	40.6
Unrealized Losses, Total	2.1	0.3
Foreign Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	117.7	18
Unrealized Losses, Less Than One Year	2.4	0.5
Fair Value, One Year or Greater	1.3	0.1
Unrealized Losses, One Year or Greater	0.3	0.2
Fair Value, Total	119	18.1
Unrealized Losses, Total	2.7	0.7
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	28.3	5.2
Unrealized Losses, Less Than One Year	0.5	0.3
Fair Value, One Year or Greater	0.4	0
Unrealized Losses, One Year or Greater	0.4	0
Fair Value, Total	28.7	5.2
Unrealized Losses, Total	0.9	0.3
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	626.4	202.5
Unrealized Losses, Less Than One Year	10.8	6.7
Fair Value, One Year or Greater	40.7	17.5
Unrealized Losses, One Year or Greater	3.6	6.6
Fair Value, Total	667.1	220
Unrealized Losses, Total	14.4	13.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	136.9	21.8
Unrealized Losses, Less Than One Year	2.6	0.3
Fair Value, One Year or Greater	8.9	0
Unrealized Losses, One Year or Greater	0.4	0
Fair Value, Total	145.8	21.8
Unrealized Losses, Total	3	0.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	207.4	74.4
Unrealized Losses, Less Than One Year	4.6	0.4
Fair Value, One Year or Greater	4.8	3
Unrealized Losses, One Year or Greater	0.2	0.2
Fair Value, Total	212.2	77.4
Unrealized Losses, Total	4.8	0.6
Asset-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	21.3	4.6
Unrealized Losses, Less Than One Year	0.1	0.4
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	21.3	4.6
Unrealized Losses, Total	0.1	0.4
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	126.8	121.1
Unrealized Losses, Less Than One Year	1.3	0.9
Fair Value, One Year or Greater	2.4	49.1
Unrealized Losses, One Year or Greater	0	0.3
Fair Value, Total	129.2	170.2
Unrealized Losses, Total	$ 1.3	$ 1.2

Investments - Allowance for Cre

Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	$ 6.4	$ 40.2	$ 6.6	$ 0
Securities for which allowance was not previously recorded	0.9	13.7	2.9	15
Securities sold during the period	(0.2)	(14.9)	(1.1)	(15.7)
Additional net increases (decreases) in existing allowance	(0.1)	(5.3)	(1.4)	27.5
Ending balance, September 30, 2021	7	33.7	7	33.7
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Additions-initial adoption of accounting standard					$ 6.9
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0.2	0.3	0.2	0
Additions-initial adoption of accounting standard					0
Securities for which allowance was not previously recorded	0	0.1	0	0.3
Securities sold during the period	0	(0.2)	0	(0.2)
Additional net increases (decreases) in existing allowance	0	0	0	0.1
Ending balance, September 30, 2021	0.2	0.2	0.2	0.2
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0	0.1	0.1	0
Additions-initial adoption of accounting standard					0
Securities for which allowance was not previously recorded	0	0	0	0.3
Securities sold during the period	0	0	0	0
Additional net increases (decreases) in existing allowance	0	0.1	(0.1)	(0.1)
Ending balance, September 30, 2021	0	0.2	0	0.2
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	6.2	39.7	6.1	0
Additions-initial adoption of accounting standard					6.8
Securities for which allowance was not previously recorded	0.7	13.6	2.7	14.4
Securities sold during the period	(0.2)	(14.7)	(1.1)	(15.5)
Additional net increases (decreases) in existing allowance	(0.1)	(6.4)	(1.1)	26.5
Ending balance, September 30, 2021	6.6	32.2	6.6	32.2
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0	0.1	0.2	0
Additions-initial adoption of accounting standard					$ 0.1
Securities for which allowance was not previously recorded	0.2	0	0.2	0
Securities sold during the period	0	0	0	0
Additional net increases (decreases) in existing allowance	0	1	(0.2)	1
Ending balance, September 30, 2021	$ 0.2	$ 1.1	$ 0.2	$ 1.1

Investments - Schedule of Compa

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 4.2	$ 18	$ 38.6	$ 133.5
Realized losses on fixed maturities and other	(5.8)	(31)	(37.1)	(128)
Credit losses on fixed maturity securities	0.8	10.5	1.5	43
Net realized gains (losses) on equity securities	1.4	(3.2)	0.3	(15.8)
Change in unrealized gains (losses) on equity securities held at the end of the period	(5.1)	10.5	30.7	(13.7)
Net realized gains (losses) on equity securities	(3.7)	7.3	31	(29.5)
Net realized investment and other gains (losses) before income taxes	(5.3)	(5.7)	32.5	(24)
Income tax (benefit) provision	0	(0.5)	0	(1.7)
Net realized investment gains (losses), net of income taxes	(5.3)	(5.2)	32.5	(22.3)
Fixed maturities
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	3.3	7.2	26.3	30.5
Realized losses on fixed maturities and other	(0.4)	(7.5)	(5)	(25.7)
Other investments
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.9	10.8	8.9	70.7
Realized losses on fixed maturities and other	(3.3)	(12.6)	(16.6)	(58.4)
Other assets
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0	0	3.4	32.3
Realized losses on fixed maturities and other	(1.4)	(0.4)	(14)	(0.9)
Credit losses on fixed maturities
Schedule of Investments [Line Items]
Credit losses on fixed maturity securities	$ (0.7)	$ (10.5)	$ (1.5)	$ (43)

Investments - Schedule of Chang

Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (23.3)	$ 42.7	$ (71.3)	$ 52.9
Income tax provision (benefit)	(4.4)	7	(13)	12.2
Net unrealized investment gains (losses), net of income taxes	(18.9)	35.7	(58.3)	40.7
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(23)	42.5	(70.7)	66.9
Other investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0	0.2	0	(14)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.3)	$ 0	$ (0.6)	$ 0

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (1.7)	$ 0.9
Operational currency exposure
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	(0.3)	0.4
Asset manager investment exposure
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	2	(0.2)
Total return strategy
Schedule of Investments [Line Items]
Foreign currency exchange, fair value	$ (3.4)	$ 0.7

Investments - Schedule of Reali

Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Gross realized investment gains	$ 7.9	$ 14.5	$ 22.9	$ 54.6
Gross realized investment losses	(12.1)	(13.1)	(30.6)	(55.4)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(4.2)	1.4	(7.7)	(0.8)
Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	0	4.2	7.1	10.1
Gross realized investment losses	(5.2)	(1.8)	(17.8)	(6.5)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	1	0	2.8	1
Gross realized investment losses	0	(0.9)	(0.8)	(1.9)
Total return strategy
Schedule of Investments [Line Items]
Gross realized investment gains	6.9	10.3	13	43.5
Gross realized investment losses	$ (6.9)	$ (10.4)	$ (12)	$ (47)

Investments - Components of Res

Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities pledged as collateral for letters of credit and other	$ 191	$ 189.4
Securities and cash on deposit supporting Lloyd's business	331.2	409.2
Total restricted investments	727.5	826.1
Securities required to be maintained at Lloyd's	397.9
Argo Re
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities and cash on deposit supporting Lloyd's business	179
Securities Deposits for Regulatory and Other Purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities on deposit for regulatory and other purposes	$ 205.3	$ 227.5

Investments - Financial Assets

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 5,313.5	$ 5,255.8
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	4,952.8	4,928.9
Fair Value, Recurring | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	4,197.3	4,107.1
Fair Value, Recurring | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	343.4	289.6
Fair Value, Recurring | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	410.2	399.8
Fair Value, Recurring | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	229.1	294.8
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	175.7	170.4
Fair Value, Recurring | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	2,023	1,981.8
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	409.2	339.7
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	493.9	508.2
Fair Value, Recurring | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	112.8	122.8
Fair Value, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	181.8	176.7
Fair Value, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	65.1	102.5
Fair Value, Recurring | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	508.6	542.6
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,052	1,069.6
Fair Value, Recurring | Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	398.3	383.5
Fair Value, Recurring | Level 1 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	398.3	383.5
Fair Value, Recurring | Level 1 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 1 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	156.3	159.2
Fair Value, Recurring | Level 1 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0.4
Fair Value, Recurring | Level 1 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	497.4	526.5
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3,871.5	3,834.8
Fair Value, Recurring | Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3,795.2	3,716.6
Fair Value, Recurring | Level 2 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	343.4	289.6
Fair Value, Recurring | Level 2 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	11.9	16.3
Fair Value, Recurring | Level 2 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	229.1	294.8
Fair Value, Recurring | Level 2 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	175.7	170.4
Fair Value, Recurring | Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	2,019.2	1,974.8
Fair Value, Recurring | Level 2 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	409.2	339.7
Fair Value, Recurring | Level 2 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	493.9	508.2
Fair Value, Recurring | Level 2 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	112.8	122.8
Fair Value, Recurring | Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 2 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	65.1	102.1
Fair Value, Recurring | Level 2 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	11.2	16.1
Fair Value, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	29.3	24.5
Fair Value, Recurring | Level 3 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3.8	7
Fair Value, Recurring | Level 3 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3.8	7
Fair Value, Recurring | Level 3 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	25.5	17.5
Fair Value, Recurring | Level 3 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Fair Value, Recurring | Level 3 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 0	$ 0

Investments - Schedule of Recon

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 24.5	$ 25.6
Transfers into Level 3	1.5	0
Transfers out of Level 3	0	0
Included in net income	6.6	(5.9)
Included in other comprehensive income	0.1	(0.5)
Purchases	0.1	5.3
Issuances	0	0
Sales	(3.5)	0
Settlements	0	0
Ending balance	29.3	24.5
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	0	0
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7	7.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	0.1	0
Included in other comprehensive income	0.1	(0.5)
Purchases	0.1	0.1
Issuances	0	0
Sales	(3.5)	0
Settlements	0	0
Ending balance	3.8	7
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	0	0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	17.5	18.2
Transfers into Level 3	1.5	0
Transfers out of Level 3	0	0
Included in net income	6.5	(5.9)
Included in other comprehensive income	0	0
Purchases	0	5.2
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	25.5	17.5
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	0	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0
Transfers into Level 3	0
Transfers out of Level 3	0
Included in net income	0
Included in other comprehensive income	0
Purchases	0
Issuances	0
Sales	0
Settlements	0
Ending balance	0	$ 0
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	$ 0

Allowance for Credit Losses - P

Allowance for Credit Losses - Premiums Receivable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Credit Loss [Abstract]
Premiums receivable	$ 701.9	$ 679.8
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2021	9.4
Current period change for estimated uncollectible premiums	0
Write-offs of uncollectible premiums receivable	(0.6)
Foreign exchange adjustments	0
Balance, September 30, 2021	$ 8.8

Allowance for Credit Losses - R

Allowance for Credit Losses - Reinsurance Recoverables (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Credit Loss [Abstract]
Reinsurance recoverables, net	$ 2,954	$ 3,009
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2021	4.1
Current period change for estimated uncollectible reinsurance	(0.3)
Write-offs of uncollectible reinsurance recoverables	0
Balance, September 30, 2021	$ 3.8

Reserves for Losses and Loss _3

Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (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
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 2,906.1	$ 2,722.7
Net AIL reserves acquired			0	27.9
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			884.9	876.9
Prior accident years			6	6.1
Losses and LAE incurred during calendar year, net of reinsurance	$ 311.7	$ 328.9	890.9	883
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior			219.7	0
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			100.5	188.2
Prior accident years			552.8	632.7
Losses and LAE payments made during current calendar year, net of reinsurance:			653.3	820.9
Change in participation interest			12.5	33
Foreign exchange adjustments			(7.3)	23
Net reserves - end of period	2,929.2	2,868.7	2,929.2	2,868.7	$ 2,906.1
Reinsurance recoverables on unpaid losses and LAE, end of period	2,510.3	2,521.3	2,510.3	2,521.3
Gross reserves - end of period	$ 5,439.5	$ 5,390	$ 5,439.5	$ 5,390	$ 5,406

Reserves for Losses and Loss _4

Reserves for Losses and Loss Adjustment Expenses - Impact from (Favorable) Unfavorable Development of Prior Accident Years' Loss and LAE Reserves on Each Reporting Segment (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	$ 6	$ 6.1
U.S. Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	(0.7)	(0.5)
International Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	0.1	(4.5)
Run-off Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total (favorable) unfavorable prior-year development	$ 6.6	$ 11.1

Disclosures about Fair Value _3

Disclosures about Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 457.1	$ 458.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	474.1	473.6
Junior subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	258.1	257.8
Junior subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	265.1	265.9
Junior subordinated debentures | Trust preferred debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	172.7	172.7
Junior subordinated debentures | Trust preferred debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	173.1	173.6
Junior subordinated debentures | Subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	85.4	85.1
Junior subordinated debentures | Subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	92	92.3
Senior unsecured fixed rate notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	140.3	140.2
Senior unsecured fixed rate notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	150.1	146.7
Floating rate loan stock | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	58.7	60.7
Floating rate loan stock | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 58.9	$ 61

Disclosures about Fair Value _4

Disclosures about Fair Value of Financial Instruments - Summary of Fair Values Measured on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	$ 474.1	$ 473.6
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	150.1	146.7
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	324	326.9
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	265.1	265.9
Junior subordinated debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	265.1	265.9
Junior subordinated debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures | Trust preferred debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	173.1	173.6
Junior subordinated debentures | Trust preferred debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures | Trust preferred debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	173.1	173.6
Junior subordinated debentures | Trust preferred debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures | Subordinated debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	92	92.3
Junior subordinated debentures | Subordinated debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Junior subordinated debentures | Subordinated debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	92	92.3
Junior subordinated debentures | Subordinated debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Senior unsecured fixed rate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	150.1	146.7
Senior unsecured fixed rate notes | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	150.1	146.7
Senior unsecured fixed rate notes | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Senior unsecured fixed rate notes | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Floating rate loan stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	58.9	61
Floating rate loan stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0	0
Floating rate loan stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	58.9	61
Floating rate loan stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	$ 0	$ 0

Shareholders' Equity (Detail)

Shareholders' Equity (Detail)	Sep. 15, 2021USD ($)	Aug. 06, 2021$ / shares	Sep. 15, 2020USD ($)	Sep. 11, 2020USD ($)	Aug. 07, 2020$ / shares	May 07, 2020$ / shares	Sep. 30, 2021USD ($)$ / shares	Sep. 30, 2020$ / shares	Sep. 30, 2021USD ($)$ / sharesshares	Sep. 30, 2020$ / shares	May 06, 2021	Dec. 31, 2020$ / shares	May 03, 2016USD ($)
Class of Stock [Line Items]
Cash dividend declared - common shares (in dollars per share)		$ 0.31				$ 0.31	$ 0.31	$ 0.31	$ 0.93	$ 0.93
Payments of dividends | $	$ 10,800,000			$ 10,900,000
Preferred shares par value (in dollars per share)							1		1			$ 1
Liquidation preference (in dollars per share)							$ 25,000		$ 25,000			$ 25,000
Depositary shares, dividends per share (in dollars per share)		0.437500			$ 0.330556
Number of shares repurchased (in shares) | shares									0
Fractional interest per one preference share											0.001
Series A Preferred Stock
Class of Stock [Line Items]
Payments of dividends | $	$ 2,600,000		$ 2,000,000
Preferred stock, dividends declared (in dollars per share)		$ 437.500			$ 330.556
Preferred stock, dividend rate (in percentage)		7.00%			7.00%
Preferred shares par value (in dollars per share)		$ 1			$ 1
Liquidation preference (in dollars per share)		$ 25,000			$ 25,000
2016 Repurchase Authorization
Class of Stock [Line Items]
Total number of shares authorized for purchase | $													$ 150,000,000
Remaining number of shares available under repurchase authorization | $							$ 53,300,000		$ 53,300,000

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,897.9	$ 1,707.7	$ 1,857.8	$ 1,763.7
Other comprehensive income (loss) before reclassifications			(47.5)	43.6
Amounts reclassified from accumulated other comprehensive loss			(9.1)	(15.2)
Net current-period other comprehensive income (loss)			(56.6)	28.4
Ending Balance	1,887.6	1,848.7	1,887.6	1,848.7
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				(2.2)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(37.9)	(22.6)
Other comprehensive income (loss) before reclassifications			0.2	(12.3)
Amounts reclassified from accumulated other comprehensive loss			0	0
Net current-period other comprehensive income (loss)			0.2	(12.3)
Ending Balance	(37.7)	(34.9)	(37.7)	(34.9)
Foreign Currency Translation Adjustments | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				0
Unrealized Holding Gains on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			105.1	33.5
Other comprehensive income (loss) before reclassifications			(49.2)	55.9
Amounts reclassified from accumulated other comprehensive loss	7.8	4	(9.1)	(15.2)
Net current-period other comprehensive income (loss)			(58.3)	40.7
Ending Balance	46.8	79.9	46.8	79.9
Unrealized Holding Gains on Securities | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				5.7
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(8.6)	(8.1)
Other comprehensive income (loss) before reclassifications			1.5	0
Amounts reclassified from accumulated other comprehensive loss			0	0
Net current-period other comprehensive income (loss)			1.5	0
Ending Balance	(7.1)	(8.1)	(7.1)	(8.1)
Defined Benefit Pension Plans | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				0
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	23.2	5.9	58.6	2.8
Ending Balance	$ 2	$ 36.9	$ 2	36.9
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				$ 5.7

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Unrealized gains and losses on securities:
Net of taxes			$ (9.1)	$ (15.2)
Unrealized Gains and Losses on Securities
Unrealized gains and losses on securities:
Net realized investment losses (gains)	$ 8.2	$ 4.5	(11)	(15.4)
(Benefit) provision for income taxes	(0.4)	(0.5)	1.9	0.2
Net of taxes	$ 7.8	$ 4	$ (9.1)	$ (15.2)

Net Income (Loss) Per Common _3

Net Income (Loss) Per Common Share - Net Income (Loss) Per Common Share on Basic and Diluted Basis (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
Earnings Per Share [Abstract]
Net income (loss)	$ 22.4	$ (23.1)	$ 122	$ (53.2)
Less: Preferred share dividends	2.6	2	7.9	2
Net income (loss) attributable to common shareholders	19.8	(25.1)	114.1	(55.2)
Net income (loss) attributable to common shareholders	$ 19.8	$ (25.1)	$ 114.1	$ (55.2)
Weighted average common shares outstanding - basic (in shares)	34,854,724	34,667,266	34,796,528	34,590,659
Effect of dilutive securities:
Equity compensation awards (in shares)	181,976	0	265,888	0
Weighted average common shares outstanding - diluted (in shares)	35,036,700	34,667,266	35,062,416	34,590,659
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.57	$ (0.72)	$ 3.28	$ (1.60)
Diluted (in dollars per share)	$ 0.56	$ (0.72)	$ 3.26	$ (1.60)

Net Income (Loss) Per Common _4

Net Income (Loss) Per Common Share - Additional Information (Detail) - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Earnings Per Share [Abstract]
Treasury shares (in shares)	11,315,889	11,315,889	11,315,889	11,315,889
Equity compensation awards with anti-dilutive effect (in shares)	26,422	49,796

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Supplemental Cash Flow [Line Items]
Total interest paid	$ 16	$ 21
Income taxes paid	30.7	46
Income taxes recovered	(1.2)	(1.6)
Income taxes paid, net	29.5	44.4
Senior unsecured fixed rate notes
Supplemental Cash Flow [Line Items]
Total interest paid	7	7
Junior subordinated debentures
Supplemental Cash Flow [Line Items]
Total interest paid	7.5	9.4
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.5	$ 4.6

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment arrangement, expense		$ 1.4	$ 1.4	$ 4.3	$ 5.9
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		11.4		$ 11.4
Restricted Stock Units (RSUs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years				1 year
Restricted Stock Units (RSUs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years				4 years
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment arrangement, expense		0.2	$ 0.2	$ 1.8	$ 0.9
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 5.6		$ 5.6
Performance Shares | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years				3 years
Performance Shares | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years				4 years
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized (in shares)	1,885,000
Expiration period	10 years

Share-based Compensation - Summ

Share-based Compensation - Summary of Restricted Share Activity (Detail) - Restricted Stock Units (RSUs)	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Shares
Outstanding, beginning balance (in shares) | shares	370,027
Granted (in shares) | shares	159,382
Vested and issued (in shares) | shares	(142,705)
Expired or forfeited (in shares) | shares	(92,039)
Outstanding, ending balance (in shares) | shares	294,665
Weighted-Average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 44.22
Granted (in dollars per share) | $ / shares	54.97
Vested and issued (in dollars per share) | $ / shares	45.94
Expired or forfeited (in dollars per share) | $ / shares	44.41
Outstanding, ending balance (in dollars per share) | $ / shares	$ 49.15

Share-based Compensation - Su_2

Share-based Compensation - Summary of Performance Shares Activity (Details) - Performance Shares	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Shares
Outstanding, beginning balance (in shares) | shares	157,847
Granted (in shares) | shares	100,291
Vested and issued (in shares) | shares	(8,987)
Expired or forfeited (in shares) | shares	(45,768)
Outstanding, ending balance (in shares) | shares	203,383
Weighted-Average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 43.10
Granted (in dollars per share) | $ / shares	54.79
Vested and issued (in dollars per share) | $ / shares	44.49
Expired or forfeited (in dollars per share) | $ / shares	48.36
Outstanding, ending balance (in dollars per share) | $ / shares	$ 47.62

Underwriting, Acquisition and_3

Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Underwriting, Acquisition and Insurance Expenses [Abstract]
Commissions	$ 88	$ 79.4	$ 236.2	$ 201.9
Other underwriting, acquisition and insurance expenses	105.1	99.1	315.7	304.3
Underwriting, acquisition and insurance expenses, total	193.1	178.5	551.9	506.2
Net deferral of policy acquisition costs	(16)	(16.6)	(21.1)	(16.4)
Total underwriting, acquisition and insurance expenses	$ 177.1	$ 161.9	$ 530.8	$ 489.8

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Deferred tax assets, change in federal tax rate	$ 7.4		$ 7.4
Net operating losses carryback period			2 years
Capital losses carryback period			3 years
Change in valuation allowance	0.8	$ 1.2	$ (1.1)	$ 1.2
Unrecognized tax benefits, period increase (decrease)	(0.4)		(2.3)
Interest on income taxes expense	0.1		(0.3)
Tax penalty expense	$ 0		$ (0.4)

Income Taxes - Schedule of Pre-

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 28.2	$ (26.5)	$ 138.1	$ (46.3)
Effective Tax Rate	20.60%	12.90%	11.70%	(14.80%)
Bermuda
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 7.2	$ 5.5	$ 14.5	$ (49.1)
Effective Tax Rate	0.00%	0.10%	0.00%	0.00%
United States
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 21.4	$ 7.2	$ 126.7	$ 62.5
Effective Tax Rate	26.80%	3.10%	20.70%	21.30%
United Kingdom
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (6.1)	$ (38.1)	$ (14.9)	$ (65.7)
Effective Tax Rate	(1.00%)	9.70%	68.40%	9.90%
Barbados
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0	$ 0	$ 0	$ 0
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Belgium
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0	$ 0.2	$ 0	$ 0.2
Effective Tax Rate	0.00%	22.80%	0.00%	25.30%
Brazil
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 5.9	$ 0.3	$ 9.8	$ 3.2
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United Arab Emirates
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.9	$ 0.4	$ 1.2	$ 1.8
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Ireland
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0	$ 0	$ (0.1)	$ (0.1)
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Italy
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0.2	$ (0.9)	$ 1.4	$ 0.7
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Malta
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (1.3)	$ (1.1)	$ (0.4)	$ 0.2
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Switzerland
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ 0	$ 0	$ (0.1)	$ 0
Effective Tax Rate	0.00%	1.00%	0.00%	0.00%

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 5.6	$ (6.5)	$ 28.4	$ 2.8
Tax effect of:
Nontaxable investment income	(0.1)	(0.2)	(0.4)	(0.5)
Foreign exchange adjustments	0.2	0.7	(0.3)	0.1
Goodwill	0	0	0	1
Withholding taxes	0	0	0.1	0.1
Prior period adjustments	0	0.9	0	0.9
Change in uncertain tax position liability	(0.4)	0	(2.3)	0
Change in valuation allowance	0.8	1.2	(1.1)	1.2
Impact of change in tax rate related to Finance Act 2021	0	0	(7.4)	0
Other	(0.3)	0.5	(0.9)	1.3
Income tax provision (benefit)	$ 5.8	$ (3.4)	$ 16.1	$ 6.9

Commitments and Contingencies (

Commitments and Contingencies (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Commitments and Contingencies Disclosure [Abstract]
Contractual commitments related to its limited partnership investments (up to)	$ 63
Contractual commitments period (not to exceed)	12 years

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)segment	Dec. 31, 2020USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Assets associated with trade capital providers | $	$ 597.5	$ 825.9

Segment Information - Revenue a

Segment Information - Revenue and Income (Loss) Before Income Taxes for Each 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]
Earned premiums	$ 487.5	$ 445.5	$ 1,423.9	$ 1,313.9
Net investment income	46.1	42	143.2	79
Net realized investment gains (losses)	(5.3)	(5.7)	32.5	(24)
Total revenue	528.3	481.8	1,599.6	1,368.9
(Loss) income before income taxes	28.2	(26.5)	138.1	(46.3)
Foreign currency exchange losses	1.3	(8.4)	(4.4)	(13.6)
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	43.2	(4.1)	140.2	23.4
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	3.7	0.5	10.5	1.1
(Loss) income before income taxes	(11)	(8.3)	(30.2)	(32.1)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	323.5	298.7	952.4	902.8
Net investment income	29.2	30.1	91.7	56.1
(Loss) income before income taxes	39.9	20.1	130.1	88.2
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	163.9	146.7	471.1	410.8
Net investment income	12.3	9.9	38.2	19
(Loss) income before income taxes	6.9	(14.7)	14.6	(54.5)
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.1	0.1	0.4	0.3
Net investment income	0.9	1.5	2.8	2.8
(Loss) income before income taxes	$ (3.6)	$ (9.5)	$ (4.5)	$ (10.3)

Segment Information - Schedule

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 487.5	$ 445.5	$ 1,423.9	$ 1,313.9
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	322.7	299.1	949.7	900.2
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	129.1	90.6	344.9	252.8
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	10.3	27.3	46.1	73.8
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	7.7	13	32.7	43.3
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 17.7	$ 15.5	$ 50.5	$ 43.8

Segment Information - Identifia

Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 10,518.1	$ 10,465.8
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,841.6	6,032.2
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,900.6	3,899.4
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	270.3	335.9
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 505.6	$ 198.3

Subsequent Event (Details)

Subsequent Event (Details) - Oct. 04, 2021 R$ in Millions, $ in Millions	BRL (R$)	USD ($)
Subsequent Event | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Seguros Brasil S.A.
Subsequent Event [Line Items]
Estimated cash received from sale	R$ 160	$ 29.8

Uncategorized Items - argo-2021

Label	Element	Value
Accounting Standards Update [Extensible Enumeration]	us-gaap_AccountingStandardsUpdateExtensibleList	Accounting Standards Update 2016-13 [Member]