Arch Capital (ACGL) 10-Q 8 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 03, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH CAPITAL GROUP LTD.
Entity Central Index Key	947,484
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		405,299,490

Unaudited Consolidated Balance

Unaudited Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments:
Fixed maturities available for sale, at fair value (amortized cost: $14,293,121 and $13,869,460)	$ 14,128,989	$ 13,876,003
Short-term investments available for sale, at fair value (amortized cost: $1,096,532 and $1,468,955)	1,096,798	1,469,042
Collateral received under securities lending, at fair value (amortized cost: $236,948 and $476,605)	236,956	476,615
Equity securities, at fair value	534,482	495,804
Other investments available for sale, at fair value (cost: $0 and $198,163)	0	264,989
Investments accounted for using the fair value option	4,111,611	4,216,237
Investments accounted for using the equity method	1,428,582	1,041,322
Total investments	21,537,418	21,840,012
Cash	526,628	606,199
Accrued investment income	114,307	113,133
Securities pledged under securities lending, at fair value (amortized cost: $229,857 and $463,181)	230,064	464,917
Premiums receivable	1,351,310	1,135,249
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,727,303	2,540,143
Contractholder receivables	2,044,322	1,978,414
Ceded unearned premiums	1,014,663	926,611
Deferred acquisition costs net	569,817	535,824
Receivable for securities sold	143,809	205,536
Goodwill and intangible assets	593,008	652,611
Other assets	1,000,471	1,053,009
Total assets	31,853,120	32,051,658
Liabilities
Reserve for losses and loss adjustment expenses	11,424,337	11,383,792
Unearned premiums	3,833,540	3,622,314
Reinsurance balances payable	411,082	323,496
Contractholder payables	2,044,322	1,978,414
Collateral held for insured obligations	257,396	240,183
Senior notes	1,733,211	1,732,884
Revolving credit agreement borrowings	572,289	816,132
Securities lending payable	236,948	476,605
Payable for securities purchased	356,583	449,186
Other liabilities	752,399	782,717
Total liabilities	21,622,107	21,805,723
Commitments and Contingencies
Redeemable noncontrolling interests	206,105	205,922
Shareholders' Equity
Non-cumulative preferred shares	780,000	872,555
Convertible non-voting common equivalent preferred shares	0	489,627
Common shares ($0.0011 par, shares issued: 569,458,341 and 549,872,226)	633	611
Additional paid-in capital	1,760,606	1,230,617
Retained earnings	9,083,202	8,562,889
Accumulated other comprehensive income (loss), net of deferred income tax	(194,157)	118,044
Common shares held in treasury, at cost (shares: 164,021,704 and 156,938,409)	(2,266,529)	(2,077,741)
Total shareholders' equity available to Arch	9,163,755	9,196,602
Non-redeemable noncontrolling interests	861,153	843,411
Total shareholders' equity	10,024,908	10,040,013
Total liabilities, noncontrolling interests and shareholders' equity	$ 31,853,120	$ 32,051,658

Unaudited Consolidated Balance3

Unaudited Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fixed maturities available for sale, at amortized cost	$ 14,293,121	$ 13,869,460
Short-term investments available for sale, at amortized cost	1,096,532	1,468,955
Collateral received under securities lending, at amortized cost	236,948	476,605
Other investments available for sale, at cost	0	198,163
Securities pledged under securities lending, at amortized cost	$ 229,857	$ 463,181
Common shares, par value per share	$ 0.0011	$ 0.0011
Common shares issued (shares)	569,458,341	549,872,226
Common shares held in treasury (shares)	164,021,704	156,938,409

Unaudited Consolidated Statemen

Unaudited Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net premiums written	$ 1,298,896	$ 1,248,695	$ 2,711,440	$ 2,524,955
Change in unearned premiums	37,867	(7,821)	(139,778)	(167,064)
Net premiums earned	1,336,763	1,240,874	2,571,662	2,357,891
Net investment income	135,668	111,124	262,392	228,998
Net realized gains (losses)	(76,611)	21,735	(187,609)	55,888
Other-than-temporary impairment losses	(470)	(1,730)	(632)	(3,537)
Less investment impairments recognized in other comprehensive income, before taxes	0	0	0	0
Net impairment losses recognized in earnings	(470)	(1,730)	(632)	(3,537)
Other underwriting income	3,874	4,822	9,223	9,455
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541	80,794
Other income (loss)	3,113	(1,994)	3,187	(2,776)
Total revenues	1,410,809	1,407,537	2,694,764	2,726,713
Expenses
Losses and loss adjustment expenses	726,153	689,860	1,363,013	1,242,430
Acquisition expenses	202,838	190,436	394,214	372,725
Other operating expenses	176,181	169,981	351,196	344,700
Corporate expenses	22,512	24,876	37,824	52,668
Amortization of intangible assets	26,472	30,824	53,208	62,118
Interest expense	30,344	28,749	60,980	57,425
Net foreign exchange (gains) losses	(53,706)	39,543	(33,985)	58,947
Total expenses	1,130,794	1,174,269	2,226,450	2,191,013
Income before income taxes	280,015	233,268	468,314	535,700
Income tax expense	(23,668)	(34,169)	(45,583)	(62,566)
Net income	256,347	199,099	422,731	473,134
Net (income) loss attributable to noncontrolling interests	(12,701)	(13,932)	(28,662)	(34,840)
Net income available to Arch	243,646	185,167	394,069	438,294
Preferred dividends	(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares	0	0	(2,710)	0
Net income available to Arch common shareholders	$ 233,243	$ 173,818	$ 370,519	$ 415,727
Net income per common share
Basic (per share)	$ 0.58	$ 0.43	$ 0.91	$ 1.03
Diluted (per share)	$ 0.56	$ 0.42	$ 0.89	$ 1
Weighted average common shares and common share equivalents outstanding
Basic (shares)	404,800,421	403,459,992	406,162,508	402,786,129
Diluted (shares)	413,111,205	417,733,938	415,460,756	417,421,896

Unaudited Consolidated Stateme5

Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive Income
Net income	$ 256,347	$ 199,099	$ 422,731	$ 473,134
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	(85,271)	92,969	(251,948)	193,761
Reclassification of net realized (gains) losses, net of income taxes, included in net income	36,643	(17,224)	99,104	(22,268)
Foreign currency translation adjustments	(12,595)	18,297	(11,313)	21,421
Comprehensive income	195,124	293,141	258,574	666,048
Net (income) loss attributable to noncontrolling interests	(12,701)	(13,932)	(28,662)	(34,840)
Other comprehensive (income) loss attributable to noncontrolling interests	1,077	76	1,750	68
Comprehensive income available to Arch	$ 183,500	$ 279,285	$ 231,662	$ 631,276

Unaudited Consolidated Stateme6

Unaudited Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Non-cumulative preferred shares	Common shares	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of deferred income tax	Unrealized appreciation (decline) in value of available-for-sale securities, net of deferred income tax	Foreign currency translation adjustments	Common shares held in treasury, at cost	Convertible non-voting common equivalent preferred shares
Balance at beginning of year, as adjusted					$ 7,996,387		$ (27,641)
Balance at beginning of year at Dec. 31, 2016		$ 772,555	$ 582	$ 531,687	7,996,701	$ (114,541)	(27,641)	$ (86,900)	$ (2,034,570)	$ 1,101,304
Cumulative effect of an accounting change at Dec. 31, 2016					(314)		0
Preferred shares redeemed		0
Preferred shares converted to common shares				611,653						(611,677)
Common shares issued, net			27
Loss on redemption of preferred shares	$ 0			0	0
All other				53,544
Net income	473,134				473,134
Net (income) loss attributable to noncontrolling interests	(34,840)				(34,840)
Preferred share dividends	(22,567)				(22,567)
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							171,493
Unrealized holding gains (losses) arising during period attributable to noncontrolling interests							0
Foreign currency translation adjustments	21,421							21,421
Foreign currency translation adjustments attributable to noncontrolling interests								68
Shares repurchased for treasury									(16,773)
Balance at end of period at Jun. 30, 2017	8,898,887	772,555	609	1,196,884	8,412,114	78,441	143,852	(65,411)	(2,051,343)	489,627
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2017	877,456
Total shareholders’ equity at Jun. 30, 2017	9,776,343
Loss on redemption of preferred shares	0
Net income	199,099
Net (income) loss attributable to noncontrolling interests	(13,932)
Preferred share dividends	(11,349)
Foreign currency translation adjustments	18,297
Balance at end of period at Jun. 30, 2017	8,898,887	772,555	609	1,196,884	8,412,114	78,441	143,852	(65,411)	(2,051,343)	489,627
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2017	877,456
Total shareholders’ equity at Jun. 30, 2017	9,776,343
Balance at beginning of year, as adjusted					8,712,683		7,606
Balance at beginning of year at Dec. 31, 2017	9,196,602	872,555	611	1,230,617	8,562,889	118,044	157,400	(39,356)	(2,077,741)	489,627
Cumulative effect of an accounting change at Dec. 31, 2017					149,794		(149,794)
Preferred shares redeemed		(92,555)
Preferred shares converted to common shares				489,608						(489,627)
Common shares issued, net			22
Loss on redemption of preferred shares	(2,710)			2,710	(2,710)
All other				37,671
Net income	422,731				422,731
Net (income) loss attributable to noncontrolling interests	(28,662)				(28,662)
Preferred share dividends	(20,840)				(20,840)
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							(152,844)
Unrealized holding gains (losses) arising during period attributable to noncontrolling interests							1,885
Foreign currency translation adjustments	(11,313)							(11,313)
Foreign currency translation adjustments attributable to noncontrolling interests								(135)
Shares repurchased for treasury	(173,600)								(188,788)
Balance at end of period at Jun. 30, 2018	9,163,755	780,000	633	1,760,606	9,083,202	(194,157)	(143,353)	(50,804)	(2,266,529)	0
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2018	861,153
Total shareholders’ equity at Jun. 30, 2018	10,024,908
Loss on redemption of preferred shares	0
Net income	256,347
Net (income) loss attributable to noncontrolling interests	(12,701)
Preferred share dividends	(10,403)
Foreign currency translation adjustments	(12,595)
Balance at end of period at Jun. 30, 2018	9,163,755	$ 780,000	$ 633	$ 1,760,606	$ 9,083,202	$ (194,157)	$ (143,353)	$ (50,804)	$ (2,266,529)	$ 0
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2018	861,153
Total shareholders’ equity at Jun. 30, 2018	$ 10,024,908

Unaudited Consolidated Stateme7

Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net income	$ 422,731	$ 473,134
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized (gains) losses	177,442	(74,985)
Net impairment losses recognized in earnings	632	3,537
Equity in net income or loss of investment funds accounted for using the equity method and other income or loss	(13,543)	(47,529)
Amortization of intangible assets	53,208	62,118
Share-based compensation	35,419	42,739
Changes in:
Reserve for losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable	(77,891)	180,342
Unearned premiums, net of ceded unearned premiums	139,778	167,064
Premiums receivable	(236,950)	(222,498)
Deferred acquisition costs	(35,111)	(53,553)
Reinsurance balances payable	88,961	50,112
Other items, net	(57,790)	(46,946)
Net cash provided by operating activities	496,886	533,535
Investing Activities
Purchases of fixed maturity investments	(16,867,570)	(19,899,326)
Purchases of equity securities	(679,663)	(400,155)
Purchases of other investments	(1,017,147)	(883,704)
Proceeds from sales of fixed maturity investments	16,090,543	19,611,680
Proceeds from sales of equity securities	622,068	473,064
Proceeds from sales, redemptions and maturities of other investments	773,298	614,494
Proceeds from redemptions and maturities of fixed maturity investments	511,448	447,941
Net settlements of derivative instruments	4,498	(5,984)
Net sales (purchases) of short-term investments	451,901	(445,203)
Change in cash collateral related to securities lending	176,304	175,693
Purchases of fixed assets	(13,242)	(11,103)
Other	49,961	33,488
Net cash provided by (used for) investing activities	102,399	(289,115)
Financing Activities
Redemption of preferred shares	(92,555)	0
Purchases of common shares under share repurchase program	(173,575)	0
Proceeds from common shares issued, net	(13,851)	(6,838)
Proceeds from borrowings	130,579	0
Repayments of borrowings	(373,000)	(72,000)
Change in cash collateral related to securities lending	(176,304)	(175,693)
Dividends paid to redeemable noncontrolling interests	(8,994)	(8,994)
Other	(4,489)	(41,698)
Preferred dividends paid	(20,840)	(22,567)
Net cash provided by (used for) financing activities	(733,029)	(327,790)
Effects of exchange rate changes on foreign currency cash and restricted cash	(10,431)	9,616
Increase (decrease) in cash and restricted cash	(144,175)	(73,754)
Cash and restricted cash, beginning of year	727,284	969,569
Cash and restricted cash, end of period	$ 583,109	$ 895,815

Unaudited Consolidated Stateme8

Unaudited Consolidated Statements of Cash Flows Unaudited Consolidated Statements of Cash Flows (Parentheticals) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Reconciliation of cash and restricted cash within the Consolidated Balance Sheets:
Cash	$ 526,628	$ 606,199
Restricted cash (included in ‘other assets’)	56,481	121,085
Cash and restricted cash	$ 583,109	$ 727,284

General

General	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
General	General Arch Capital Group Ltd. (“Arch Capital”) is a Bermuda public limited liability company which provides insurance, reinsurance and mortgage insurance on a worldwide basis through its wholly-owned subsidiaries. As used herein, the “Company” means Arch Capital and its subsidiaries. The Company’s consolidated financial statements include the results of Watford Holdings Ltd. and its wholly owned subsidiaries. See Note 11 . The interim consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). All significant intercompany transactions and balances have been eliminated in consolidation. The preparation of 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 the reported amounts of revenues and expenses during the reporting period. Actual results could differ materially from those estimates and assumptions. In the opinion of management, the accompanying unaudited interim consolidated financial statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. The results of any interim period are not necessarily indicative of the results for a full year or any future periods. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted; however, management believes that the disclosures are adequate to make the information presented not misleading. This report should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 (“2017 Form 10-K”), including the Company’s audited consolidated financial statements and related notes. The Company has reclassified the presentation of certain prior year information to conform to the current presentation. Such reclassifications had no effect on the Company’s net income, comprehensive income, shareholders’ equity or cash flows. Tabular amounts are in U.S. Dollars in thousands, except share amounts, unless otherwise noted.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	6 Months Ended
	Jun. 30, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Issued Accounting Standards Adopted The Company adopted ASU 2016-01, “Financial Instruments - Overall (Subtopic 825-10) - Recognition and Measurement of Financial Assets and Financial Liabilities,” which enhances the reporting model for financial instruments and provides improved financial information to readers of the financial statements. Among other provisions focused on improving the recognition and measurement of financial instruments, the ASU significantly changes the income statement impact of equity instruments and the recognition of changes in fair value of financial liabilities attributable to an entity's own credit risk when the fair value option is elected. The ASU requires equity instruments that do not result in consolidation and are not accounted for under the equity method to be measured at fair value with any changes in fair value recognized in net income rather than other comprehensive income. Upon adoption of this ASU, the Company recorded a cumulative effect adjustment of $149.8 million in retained earnings and an offsetting decrease in accumulated other comprehensive income. The adoption of this ASU did not have a material impact on the Company's financial position, cash flows, or total comprehensive income, but may increase volatility in the Company's results of operations in future periods. The Company adopted Financial Accounting Standards Board (“FASB”) Accounting Standard Update (“ASU”) 2014-09, “Revenue from Contracts with Customers (Topic 606),” which creates a new comprehensive revenue recognition standard that serves as a single source of revenue guidance for all companies in all industries. The guidance applies to all companies that either enter into contracts with customers to transfer goods or services or enter into contracts for the transfer of non-financial assets, unless those contracts are within the scope of other standards, such as insurance contracts or financial instruments. The ASU also requires enhanced disclosures about revenue. The Company adopted the ASU using the modified retrospective method, whereby the cumulative effect of adoption was recognized as an adjustment to retained earnings at the date of initial application. The impact of the adoption of this ASU was not material, mostly because the accounting for insurance contracts is outside of the scope of ASU 2014-09. The Company adopted ASU 2016-18, “Statement of Cash Flows (Topic 230) - Restricted Cash,” which requires that restricted cash and restricted cash equivalents be included with cash and cash equivalents in the reconciliation of beginning and ending cash on the statements of cash flows. As a result, transfers between cash and cash equivalents and restricted cash and restricted cash equivalents will no longer be presented on the statement of cash flows. The revised presentation required in this ASU is reflected in the Company’s consolidated statements of cash flows for both periods presented. The adoption of this ASU did not have any effect on the Company’s results of operations, financial position or comprehensive income. Recently Issued Accounting Standards Not Yet Adopted For information regarding accounting standards that the Company has not yet adopted, see note 3(q), “Significant Accounting Policies—Recent Accounting Pronouncements,” of the notes to consolidated financial statements in the Company’s 2017 Form 10-K.

Share Transactions

Share Transactions	6 Months Ended
	Jun. 30, 2018
Stockholders' Equity Note [Abstract]
Share Transactions	Share Transactions Three-For-One Common Share Split In May 2018, shareholders approved a proposal to amend the memorandum of association by sub-dividing the authorized common shares of Arch Capital to effect a three -for-one split of Arch Capital’s common shares. The share split changed the Company’s authorized common shares to 1.8 billion common shares ( 600 million previously), with a par value of $.0011 per share ( $.0033 previously). Information pertaining to the composition of the Company’s shareholders’ equity accounts, shares and earnings per share has been retroactively restated in the accompanying financial statements and notes to the consolidated financial statements to reflect the share split. Share-Based Compensation During the 2018 second quarter, the Company granted 2,199,656 stock options, 1,264,931 restricted shares and units and 705,345 performance shares and units to certain employees and directors with weighted average grant-date fair values of $7.43 , $26.56 and $24.65 , respectively. During the 2017 second quarter, the Company granted 1,477,398 stock options and 1,534,908 restricted shares and units to certain employees and directors with weighted average grant-date fair values of $8.22 and $32.09 , respectively. The stock options were valued at the grant date using the Black-Scholes option pricing model. Such values are being amortized over the respective substantive vesting period. For awards granted to retirement-eligible employees where no service is required for the employee to retain the award, the grant date fair value is immediately recognized as compensation expense at the grant date because the employee is able to retain the award without continuing to provide service. For employees near retirement eligibility, attribution of compensation cost is recognized over the period from the grant date to the retirement eligibility date. Share Repurchases The board of directors of Arch Capital has authorized the investment in Arch Capital’s common shares through a share repurchase program. Since the inception of the share repurchase program, Arch Capital has repurchased 382.2 million common shares for an aggregate purchase price of $3.86 billion . For the six months ended June 30, 2018 , Arch Capital repurchased 6,522,645 shares under the share repurchase program with an aggregate purchase price of $173.6 million . Arch Capital did not repurchase any shares under the share repurchase program during the six months ended June 30, 2017 . At June 30, 2018 , $272.9 million of share repurchases were available under the program, which may be effected from time to time in open market or privately negotiated transactions through December 31, 2019. The timing and amount of the repurchase transactions under this program will depend on a variety of factors, including market conditions and corporate and regulatory considerations. Conversion of Convertible Non-Voting Common Equivalent Preferred Shares In March 2018, Arch Capital completed an underwritten public secondary offering of 17,022,600 common shares (split adjusted) by American International Group, Inc. (“AIG”) following transfer of 567,420 Series D convertible non-voting common equivalent preferred shares (“Series D Preferred Shares”). Proceeds from the sale of common shares pursuant to the public offering were received by AIG. At June 30, 2018 , no Series D Preferred Shares were outstanding. Series C Preferred Shares On January 2, 2018, Arch Capital redeemed all outstanding 6.75% Series C non-cumulative preferred shares. The preferred shares were redeemed at a redemption price equal to $25 per share, plus all declared and unpaid dividends to (but excluding) the redemption date. In accordance with GAAP, following the redemption, original issuance costs related to such shares have been removed from additional paid-in capital and recorded as a “loss on redemption of preferred shares.” Such adjustment had no impact on total shareholders’ equity or cash flows.

Earnings Per Common Share

Earnings Per Common Share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share The following table sets forth the computation of basic and diluted earnings per common share: Three Months Ended Six Months Ended June 30, June 30, 2018 2017 2018 2017 Numerator: Net income $ 256,347 $ 199,099 $ 422,731 $ 473,134 Amounts attributable to noncontrolling interests (12,701 ) (13,932 ) (28,662 ) (34,840 ) Net income available to Arch 243,646 185,167 394,069 438,294 Preferred dividends (10,403 ) (11,349 ) (20,840 ) (22,567 ) Loss on redemption of preferred shares — — (2,710 ) — Net income available to Arch common shareholders $ 233,243 $ 173,818 $ 370,519 $ 415,727 Denominator: Weighted average common shares outstanding 404,800,421 369,027,702 399,485,135 366,436,407 Series D preferred shares (1) — 34,432,290 6,677,373 36,349,722 Weighted average common shares and common share equivalents outstanding — basic 404,800,421 403,459,992 406,162,508 402,786,129 Effect of dilutive common share equivalents: Nonvested restricted shares 1,575,749 4,448,889 1,837,356 4,670,889 Stock options (2) 6,735,035 9,825,057 7,460,892 9,964,878 Weighted average common shares and common share equivalents outstanding — diluted 413,111,205 417,733,938 415,460,756 417,421,896 Earnings per common share: Basic $ 0.58 $ 0.43 $ 0.91 $ 1.03 Diluted $ 0.56 $ 0.42 $ 0.89 $ 1.00 (1) Such shares are convertible non-voting common equivalent preferred shares issued in connection with the UGC acquisition. See Note 3 . (2) Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For the 2018 second quarter and 2017 second quarter , the number of stock options excluded were 5,350,733 and 1,499,997 , respectively. For the six months ended June 30, 2018 and 2017 , the number of stock options excluded were 5,372,789 and 2,292,228 , respectively.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information The Company classifies its businesses into three underwriting segments — insurance, reinsurance and mortgage — and two other operating segments — ‘other’ and corporate (non-underwriting). The Company determined its reportable segments using the management approach described in accounting guidance regarding disclosures about segments of an enterprise and related information. The accounting policies of the segments are the same as those used for the preparation of the Company’s consolidated financial statements. Intersegment business is allocated to the segment accountable for the underwriting results. The Company’s insurance, reinsurance and mortgage segments each have managers who are responsible for the overall profitability of their respective segments and who are directly accountable to the Company’s chief operating decision makers, the President and Chief Executive Officer of Arch Capital, and the Chief Financial Officer of Arch Capital. The chief operating decision makers do not assess performance, measure return on equity or make resource allocation decisions on a line of business basis. Management measures segment performance for its three underwriting segments based on underwriting income or loss. The Company does not manage its assets by underwriting segment, with the exception of goodwill and intangible assets, and, accordingly, investment income is not allocated to each underwriting segment. The insurance segment consists of the Company’s insurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: construction and national accounts; excess and surplus casualty; lenders products; professional lines; programs; property, energy, marine and aviation; travel, accident and health; and other (consisting of alternative markets, excess workers' compensation and surety business). The reinsurance segment consists of the Company’s reinsurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: casualty; marine and aviation; other specialty; property catastrophe; property excluding property catastrophe (losses on a single risk, both excess of loss and pro rata); and other (consisting of life reinsurance, casualty clash and other). The mortgage segment includes the Company’s U.S. and international mortgage insurance and reinsurance operations as well as government sponsored enterprise (“GSE”) credit-risk sharing transactions. Arch Mortgage Insurance Company and United Guaranty Residential Insurance Company (combined “Arch MI U.S.”) are approved as eligible mortgage insurers by Federal National Mortgage Association (“Fannie Mae”) and Federal Home Loan Mortgage Corporation (“Freddie Mac”), each a GSE. The corporate (non-underwriting) segment results include net investment income, other income (loss), corporate expenses, UGC transaction costs and other, interest expense, items related to the Company’s non-cumulative preferred shares, net realized gains or losses, net impairment losses included in earnings, equity in net income or loss of investment funds accounted for using the equity method, net foreign exchange gains or losses and income taxes. Such amounts exclude the results of the ‘other’ segment. The ‘other’ segment includes the results of Watford Re (see Note 11 ). Watford Re has its own management and board of directors that is responsible for the overall profitability of the ‘other’ segment. For the ‘other’ segment, performance is measured based on net income or loss. The following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income or loss to net income available to Arch common shareholders: Three Months Ended June 30, 2018 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 769,372 $ 490,327 $ 330,990 $ 1,591,202 $ 175,175 $ 1,696,544 Premiums ceded (245,265 ) (136,247 ) (50,867 ) (432,892 ) (34,589 ) (397,648 ) Net premiums written 524,107 354,080 280,123 1,158,310 140,586 1,298,896 Change in unearned premiums 22,342 (13,762 ) 10,355 18,935 18,932 37,867 Net premiums earned 546,449 340,318 290,478 1,177,245 159,518 1,336,763 Other underwriting income (loss) — (129 ) 3,315 3,186 688 3,874 Losses and loss adjustment expenses (357,465 ) (229,956 ) (21,591 ) (609,012 ) (117,141 ) (726,153 ) Acquisition expenses (90,670 ) (50,142 ) (27,737 ) (168,549 ) (34,289 ) (202,838 ) Other operating expenses (92,680 ) (35,678 ) (38,729 ) (167,087 ) (9,094 ) (176,181 ) Underwriting income (loss) $ 5,634 $ 24,413 $ 205,736 235,783 (318 ) 235,465 Net investment income 107,761 27,907 135,668 Net realized gains (losses) (59,545 ) (17,066 ) (76,611 ) Net impairment losses recognized in earnings (470 ) — (470 ) Equity in net income (loss) of investment funds accounted for using the equity method 8,472 — 8,472 Other income (loss) 3,113 — 3,113 Corporate expenses (2) (15,604 ) — (15,604 ) UGC transaction costs and other (2) (6,908 ) — (6,908 ) Amortization of intangible assets (26,472 ) — (26,472 ) Interest expense (26,058 ) (4,286 ) (30,344 ) Net foreign exchange gains (losses) 46,211 7,495 53,706 Income before income taxes 266,283 13,732 280,015 Income tax expense (23,644 ) (24 ) (23,668 ) Net income 242,639 13,708 256,347 Dividends attributable to redeemable noncontrolling interests — (4,585 ) (4,585 ) Amounts attributable to nonredeemable noncontrolling interests — (8,116 ) (8,116 ) Net income available to Arch 242,639 1,007 243,646 Preferred dividends (10,403 ) — (10,403 ) Net income available to Arch common shareholders $ 232,236 $ 1,007 $ 233,243 Underwriting Ratios Loss ratio 65.4 % 67.6 % 7.4 % 51.7 % 73.4 % 54.3 % Acquisition expense ratio 16.6 % 14.7 % 9.5 % 14.3 % 21.5 % 15.2 % Other operating expense ratio 17.0 % 10.5 % 13.3 % 14.2 % 5.7 % 13.2 % Combined ratio 99.0 % 92.8 % 30.2 % 80.2 % 100.6 % 82.7 % Goodwill and intangible assets $ 20,724 $ — $ 564,634 $ 585,358 $ 7,650 $ 593,008 (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Three Months Ended June 30, 2017 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 743,902 $ 453,186 $ 336,226 $ 1,533,142 $ 152,813 $ 1,609,659 Premiums ceded (247,446 ) (115,262 ) (62,314 ) (424,850 ) (12,410 ) (360,964 ) Net premiums written 496,456 337,924 273,912 1,108,292 140,403 1,248,695 Change in unearned premiums 21,118 (23,222 ) (16,068 ) (18,172 ) 10,351 (7,821 ) Net premiums earned 517,574 314,702 257,844 1,090,120 150,754 1,240,874 Other underwriting income (loss) — (279 ) 4,277 3,998 824 4,822 Losses and loss adjustment expenses (350,939 ) (207,606 ) (20,694 ) (579,239 ) (110,621 ) (689,860 ) Acquisition expenses (78,872 ) (51,151 ) (25,666 ) (155,689 ) (34,747 ) (190,436 ) Other operating expenses (92,267 ) (36,711 ) (32,150 ) (161,128 ) (8,853 ) (169,981 ) Underwriting income (loss) $ (4,504 ) $ 18,955 $ 183,611 198,062 (2,643 ) 195,419 Net investment income 92,520 18,604 111,124 Net realized gains (losses) 18,046 3,689 21,735 Net impairment losses recognized in earnings (1,730 ) — (1,730 ) Equity in net income (loss) of investment funds accounted for using the equity method 32,706 — 32,706 Other income (loss) (1,994 ) — (1,994 ) Corporate expenses (2) (22,201 ) — (22,201 ) UGC transaction costs and other (2) (2,675 ) — (2,675 ) Amortization of intangible assets (30,824 ) — (30,824 ) Interest expense (25,912 ) (2,837 ) (28,749 ) Net foreign exchange gains (losses) (37,821 ) (1,722 ) (39,543 ) Income before income taxes 218,177 15,091 233,268 Income tax expense (34,169 ) — (34,169 ) Net income 184,008 15,091 199,099 Dividends attributable to redeemable noncontrolling interests — (4,586 ) (4,586 ) Amounts attributable to nonredeemable noncontrolling interests — (9,346 ) (9,346 ) Net income available to Arch 184,008 1,159 185,167 Preferred dividends (11,349 ) — (11,349 ) Net income available to Arch common shareholders $ 172,659 $ 1,159 $ 173,818 Underwriting Ratios Loss ratio 67.8 % 66.0 % 8.0 % 53.1 % 73.4 % 55.6 % Acquisition expense ratio 15.2 % 16.3 % 10.0 % 14.3 % 23.0 % 15.3 % Other operating expense ratio 17.8 % 11.7 % 12.5 % 14.8 % 5.9 % 13.7 % Combined ratio 100.8 % 94.0 % 30.5 % 82.2 % 102.3 % 84.6 % Goodwill and intangible assets $ 24,480 $ 609 $ 680,236 $ 705,325 $ 7,650 $ 712,975 (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Six Months Ended June 30, 2018 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 1,592,750 $ 1,067,810 $ 652,168 $ 3,312,807 $ 389,045 $ 3,534,758 Premiums ceded (492,445 ) (331,977 ) (97,004 ) (921,505 ) (68,907 ) (823,318 ) Net premiums written 1,100,305 735,833 555,164 2,391,302 320,138 2,711,440 Change in unearned premiums (15,119 ) (116,343 ) 15,556 (115,906 ) (23,872 ) (139,778 ) Net premiums earned 1,085,186 619,490 570,720 2,275,396 296,266 2,571,662 Other underwriting income (loss) — 1,103 6,731 7,834 1,389 9,223 Losses and loss adjustment expenses (711,195 ) (371,631 ) (65,057 ) (1,147,883 ) (215,130 ) (1,363,013 ) Acquisition expenses (175,839 ) (98,461 ) (54,304 ) (328,604 ) (65,610 ) (394,214 ) Other operating expenses (184,654 ) (71,249 ) (77,500 ) (333,403 ) (17,793 ) (351,196 ) Underwriting income (loss) $ 13,498 $ 79,252 $ 380,590 473,340 (878 ) 472,462 Net investment income 208,004 54,388 262,392 Net realized gains (losses) (171,404 ) (16,205 ) (187,609 ) Net impairment losses recognized in earnings (632 ) — (632 ) Equity in net income (loss) of investment funds accounted for using the equity method 36,541 — 36,541 Other income (loss) 3,187 — 3,187 Corporate expenses (2) (30,086 ) — (30,086 ) UGC transaction costs and other (2) (7,738 ) — (7,738 ) Amortization of intangible assets (53,208 ) — (53,208 ) Interest expense (51,965 ) (9,015 ) (60,980 ) Net foreign exchange gains (losses) 31,172 2,813 33,985 Income before income taxes 437,211 31,103 468,314 Income tax expense (45,556 ) (27 ) (45,583 ) Net income 391,655 31,076 422,731 Dividends attributable to redeemable noncontrolling interests — (9,170 ) (9,170 ) Amounts attributable to nonredeemable noncontrolling interests — (19,492 ) (19,492 ) Net income available to Arch 391,655 2,414 394,069 Preferred dividends (20,840 ) — (20,840 ) Loss on redemption of preferred shares (2,710 ) — (2,710 ) Net income available to Arch common shareholders $ 368,105 $ 2,414 $ 370,519 Underwriting Ratios Loss ratio 65.5 % 60.0 % 11.4 % 50.4 % 72.6 % 53.0 % Acquisition expense ratio 16.2 % 15.9 % 9.5 % 14.4 % 22.1 % 15.3 % Other operating expense ratio 17.0 % 11.5 % 13.6 % 14.7 % 6.0 % 13.7 % Combined ratio 98.7 % 87.4 % 34.5 % 79.5 % 100.7 % 82.0 % (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Six Months Ended June 30, 2017 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 1,526,183 $ 928,968 $ 684,849 $ 3,139,828 $ 306,933 $ 3,267,649 Premiums ceded (481,541 ) (281,354 ) (136,239 ) (898,962 ) (22,844 ) (742,694 ) Net premiums written 1,044,642 647,614 548,610 2,240,866 284,089 2,524,955 Change in unearned premiums (21,422 ) (88,061 ) (46,243 ) (155,726 ) (11,338 ) (167,064 ) Net premiums earned 1,023,220 559,553 502,367 2,085,140 272,751 2,357,891 Other underwriting income (loss) — (585 ) 8,400 7,815 1,640 9,455 Losses and loss adjustment expenses (683,580 ) (313,060 ) (49,759 ) (1,046,399 ) (196,031 ) (1,242,430 ) Acquisition expenses (153,740 ) (97,298 ) (54,432 ) (305,470 ) (67,255 ) (372,725 ) Other operating expenses (180,393 ) (74,244 ) (74,020 ) (328,657 ) (16,043 ) (344,700 ) Underwriting income (loss) $ 5,507 $ 74,366 $ 332,556 412,429 (4,938 ) 407,491 Net investment income 188,332 40,666 228,998 Net realized gains (losses) 46,558 9,330 55,888 Net impairment losses recognized in earnings (3,537 ) — (3,537 ) Equity in net income (loss) of investment funds accounted for using the equity method 80,794 — 80,794 Other income (loss) (2,776 ) — (2,776 ) Corporate expenses (2) (34,409 ) — (34,409 ) UGC transaction costs and other (2) (18,259 ) — (18,259 ) Amortization of intangible assets (62,118 ) — (62,118 ) Interest expense (51,668 ) (5,757 ) (57,425 ) Net foreign exchange gains (losses) (57,666 ) (1,281 ) (58,947 ) Income before income taxes 497,680 38,020 535,700 Income tax expense (62,566 ) — (62,566 ) Net income 435,114 38,020 473,134 Dividends attributable to redeemable noncontrolling interests — (9,170 ) (9,170 ) Amounts attributable to nonredeemable noncontrolling interests — (25,670 ) (25,670 ) Net income available to Arch 435,114 3,180 438,294 Preferred dividends (22,567 ) — (22,567 ) Net income available to Arch common shareholders $ 412,547 $ 3,180 $ 415,727 Underwriting Ratios Loss ratio 66.8 % 55.9 % 9.9 % 50.2 % 71.9 % 52.7 % Acquisition expense ratio 15.0 % 17.4 % 10.8 % 14.6 % 24.7 % 15.8 % Other operating expense ratio 17.6 % 13.3 % 14.7 % 15.8 % 5.9 % 14.6 % Combined ratio 99.4 % 86.6 % 35.4 % 80.6 % 102.5 % 83.1 % (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’

Reserve for Losses and Loss Adj

Reserve for Losses and Loss Adjustment Expenses	6 Months Ended
	Jun. 30, 2018
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses	Reserve for Losses and Loss Adjustment Expenses The following table represents an analysis of losses and loss adjustment expenses and a reconciliation of the beginning and ending reserve for losses and loss adjustment expenses: Three Months Ended Six Months Ended June 30, June 30, 2018 2017 2018 2017 Reserve for losses and loss adjustment expenses at beginning of period $ 11,496,205 $ 10,296,821 $ 11,383,792 $ 10,200,960 Unpaid losses and loss adjustment expenses recoverable 2,446,990 2,095,130 2,464,910 2,083,575 Net reserve for losses and loss adjustment expenses at beginning of period 9,049,215 8,201,691 8,918,882 8,117,385 Net incurred losses and loss adjustment expenses relating to losses occurring in: Current year 790,742 759,261 1,478,627 1,395,037 Prior years (64,589 ) (69,401 ) (115,614 ) (152,607 ) Total net incurred losses and loss adjustment expenses 726,153 689,860 1,363,013 1,242,430 Retroactive reinsurance transaction (1) (420,404 ) — (420,404 ) — Net foreign exchange (gains) losses (118,542 ) 75,295 (74,528 ) 106,574 Net paid losses and loss adjustment expenses relating to losses occurring in: Current year (59,022 ) (80,499 ) (95,022 ) (115,502 ) Prior years (404,812 ) (482,046 ) (919,353 ) (946,586 ) Total net paid losses and loss adjustment expenses (463,834 ) (562,545 ) (1,014,375 ) (1,062,088 ) Net reserve for losses and loss adjustment expenses at end of period 8,772,588 8,404,301 8,772,588 8,404,301 Unpaid losses and loss adjustment expenses recoverable 2,651,749 2,116,210 2,651,749 2,116,210 Reserve for losses and loss adjustment expenses at end of period $ 11,424,337 $ 10,520,511 $ 11,424,337 $ 10,520,511 (1) During the 2018 second quarter, a subsidiary of the Company entered into a retroactive reinsurance transaction with a third party reinsurer to reinsure run-off liabilities associated with certain discontinued U.S. specialty casualty and program exposures. Development on Prior Year Loss Reserves 2018 Second Quarter During the 2018 second quarter , the Company recorded net favorable development on prior year loss reserves of $64.6 million , which consisted of $33.0 million from the reinsurance segment, $6.1 million from the insurance segment, $23.3 million from the mortgage segment and $2.2 million from the ‘other’ segment. The reinsurance segment’s net favorable development of $33.0 million , or 9.7 loss ratio points, for the 2018 second quarter consisted of $22.2 million from short-tailed lines and $10.8 million from long-tailed and medium-tailed lines. Favorable development in short-tailed lines included $19.3 million from property catastrophe and property other than property catastrophe reserves, across most underwriting years ( i.e. , all premiums and losses attributable to contracts having an inception or renewal date within the given twelve-month period), reflecting lower levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during the period. Favorable development in long-tailed and medium-tailed lines reflected reductions in marine reserves of $3.8 million , across most accident years, and in casualty reserves of $6.9 million based on varying levels of reported and paid claims activity, primarily from the 2003 to 2010 underwriting years. The insurance segment’s net favorable development of $6.1 million , or 1.1 loss ratio points, for the 2018 second quarter consisted of $13.9 million of net favorable development in short-tailed lines and $14.3 million of net favorable development in long-tailed lines, partially offset by $22.1 million of net adverse development in medium-tailed lines. Net favorable development in short-tailed lines primarily resulted from property (including special risk other than marine) reserves from the 2010 to 2017 accident years ( i.e. , the year in which a loss occurred) while net favorable development in long-tailed lines primarily resulted from reductions in executive assurance reserves of $6.9 million , primarily from the 2007 to 2011 accident years, and in healthcare reserves of $4.9 million , primarily from the 2003 accident year. Net adverse development in medium-tailed lines reflected $11.6 million of adverse development in program business, primarily driven by a few inactive programs that were non-renewed in 2015 and early in 2016 and $18.0 million of adverse development on contract binding business, primarily from the 2014 to 2016 accident years. Such amounts were partially offset by $7.6 million of net favorable development in other medium-tailed lines, including professional liability and surety business, across most accident years. The mortgage segment’s net favorable development was $23.3 million , or 8.0 loss ratio points, for the 2018 second quarter . The 2018 second quarter development was primarily driven by continued lower than expected claim rates on first lien business and subrogation recoveries on second lien business at Arch MI U.S. 2017 Second Quarter During the 2017 second quarter , the Company recorded net favorable development on prior year loss reserves of $69.4 million , which consisted of $39.5 million from the reinsurance segment, $2.0 million from the insurance segment, $29.8 million from the mortgage segment and adverse development of $1.9 million from the ‘other’ segment. The reinsurance segment’s net favorable development of $39.5 million , or 12.6 loss ratio points, for the 2017 second quarter consisted of $28.1 million from short-tailed lines and $11.4 million from long-tailed and medium-tailed lines. Favorable development in short-tailed lines included $16.9 million from property catastrophe and property other than property catastrophe reserves, across most underwriting years, reflecting lower levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during the period. Favorable development in long-tailed and medium-tailed lines reflected reductions in casualty reserves of $9.0 million based on varying levels of reported and paid claims activity, primarily from the 2002 to 2004 underwriting years, and favorable development in marine reserves of $2.4 million across most underwriting years. The insurance segment’s net favorable development of $2.0 million , or 0.4 loss ratio points, for the 2017 second quarter consisted of $5.3 million of net favorable development in short-tailed lines, partially offset by $3.3 million of net adverse development in medium-tailed and long-tailed lines. Net favorable development in short-tailed lines primarily resulted from property (including special risk other than marine) reserves from the 2012 to 2016 accident years. Net adverse development in medium-tailed and long-tailed lines reflected $12.2 million of adverse development on programs, primarily on a small number of programs in the 2014 and 2015 accident years, and $8.9 million on construction reserves across various accident years. Such amounts were partially offset by net favorable development of $17.8 million in other medium-tailed lines, primarily in professional liability with $12.1 million of favorable development across most accident years, and surety with $3.6 million of favorable development. The mortgage segment’s net favorable development was $29.8 million , or 11.5 loss ratio points, for the 2017 second quarter . The 2017 second quarter development was primarily driven by continued lower than expected claim rates on first lien business and subrogation recoveries on second lien business at Arch MI U.S. Six Months Ended June 30, 2018 During the six months ended June 30, 2018 , the Company recorded net favorable development on prior year loss reserves of $115.6 million , which consisted of $69.6 million from the reinsurance segment, $8.2 million from the insurance segment, $36.3 million from the mortgage segment and $1.6 million from the ‘other’ segment. The reinsurance segment’s net favorable development of $69.6 million , or 11.2 loss ratio points, for the 2018 period consisted of $51.1 million from short-tailed lines and $18.5 million from long-tailed and medium-tailed lines. Favorable development in short-tailed lines included $40.4 million from property catastrophe and property other than property catastrophe reserves, across most underwriting years, reflecting lower levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during the period. Favorable development in long-tailed and medium-tailed lines reflected reductions in casualty reserves of $8.1 million based on varying levels of reported and paid claims activity, primarily from the 2002 to 2010 underwriting years, and favorable development in marine reserves of $10.0 million across most underwriting years. The insurance segment’s net favorable development of $8.2 million , or 0.8 loss ratio points, for the 2018 period consisted of $22.7 million of net favorable development in short-tailed lines and $17.2 million of net favorable development in long-tailed lines, partially offset by $31.7 million of net adverse development in medium-tailed lines. Net favorable development in short-tailed lines primarily resulted from property (including special risk other than marine) reserves from the 2010 and 2017 accident years. Net favorable development in long-tailed lines reflected net reductions in executive assurance reserves of $7.5 million , primarily from the 2008 to 2015 accident years, and in healthcare reserves of $7.0 million , primarily from the 2003 accident year. Net adverse development in medium-tailed lines reflected $21.9 million of adverse development in program business, primarily driven by a few inactive programs that were non-renewed in 2015 and early in 2016 and $25.6 million of adverse development on contract binding business, primarily from the 2014 to 2016 accident years. Such amounts were partially offset by $15.8 million of net favorable development in other medium-tailed lines, including professional liability and surety business, across most accident years. The mortgage segment’s net favorable development was $36.3 million , or 6.4 loss ratio points, for the 2018 period. The 2018 development was primarily driven by continued lower than expected claim rates on first lien business and subrogation recoveries on second lien business at Arch MI U.S. Six Months Ended June 30, 2017 During the six months ended June 30, 2017 , the Company recorded net favorable development on prior year loss reserves of $152.6 million , which consisted of $96.8 million from the reinsurance segment, $4.1 million from the insurance segment, $53.4 million from the mortgage segment and adverse development of $1.7 million from the ‘other’ segment. The reinsurance segment’s net favorable development of $96.8 million , or 17.3 loss ratio points, for the 2017 period consisted of $68.9 million from short-tailed lines and $27.9 million from long-tailed and medium-tailed lines. Favorable development in short-tailed lines included $51.0 million from property catastrophe and property other than property catastrophe reserves, across most underwriting years, reflecting lower levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during the period. Favorable development in long-tailed and medium-tailed lines reflected reductions in casualty reserves of $15.6 million based on varying levels of reported and paid claims activity, primarily from the 2002 to 2013 underwriting years, and favorable development in marine reserves of $12.3 million across most underwriting years. The insurance segment’s net favorable development of $4.1 million , or 0.4 loss ratio points, for the 2017 period consisted of $7.2 million of net favorable development in short-tailed lines and $6.6 million of net favorable development in long-tailed lines, partially offset by $9.7 million of net adverse development in medium-tailed lines. Net favorable development in short-tailed lines primarily resulted from property (including special risk other than marine) reserves from the 2011 to 2016 accident years. Net favorable development in long-tailed lines reflected net reductions in executive assurance reserves from the 2008 to 2014 accident years and reductions in healthcare reserves across various accident years, partially offset by $13.4 million of net adverse development on construction reserves across various accident years. Net adverse development in medium-tailed lines primarily resulted from an increase in programs of $26.4 million stemming in part from development on a small number of programs in the 2013 to 2015 accident years, partially offset by net favorable development of $16.7 million in other medium-tailed lines, primarily in professional liability and surety. The mortgage segment’s net favorable development was $53.4 million , or 10.6 loss ratio points, for the 2017 period. The 2017 development was primarily driven by continued lower than expected claim rates on first lien business and subrogation recoveries on second lien business at Arch MI U.S.

Investment Information

Investment Information	6 Months Ended
	Jun. 30, 2018
Disclosure Investment Information [Abstract]
Investment Information	Investment Information At June 30, 2018 , total investable assets of $21.82 billion included $19.17 billion held by the Company and $2.65 billion attributable to Watford Re. Available For Sale Investments The following table summarizes the fair value and cost or amortized cost of the Company’s securities classified as available for sale: Estimated Fair Value Gross Unrealized Gains Gross Unrealized Losses Cost or Amortized Cost OTTI Unrealized Losses (2) June 30, 2018 Fixed maturities (1): Corporate bonds $ 5,535,763 $ 7,199 $ (103,978 ) $ 5,632,542 $ (69 ) Mortgage backed securities 488,699 1,699 (4,742 ) 491,742 (15 ) Municipal bonds 1,431,256 7,371 (22,047 ) 1,445,932 — Commercial mortgage backed securities 590,198 776 (13,222 ) 602,644 — U.S. government and government agencies 2,788,272 6,216 (19,577 ) 2,801,633 — Non-U.S. government securities 1,692,783 17,124 (27,480 ) 1,703,139 — Asset backed securities 1,820,489 3,310 (16,574 ) 1,833,753 — Total 14,347,460 43,695 (207,620 ) 14,511,385 (84 ) Equity securities (3) Other investments — — — — — Short-term investments 1,096,798 338 (72 ) 1,096,532 — Total $ 15,444,258 $ 44,033 $ (207,692 ) $ 15,607,917 $ (84 ) December 31, 2017 Fixed maturities (1): Corporate bonds $ 4,434,439 $ 30,943 $ (32,340 ) $ 4,435,836 $ (73 ) Mortgage backed securities 316,141 1,640 (2,561 ) 317,062 (15 ) Municipal bonds 2,158,840 20,285 (12,308 ) 2,150,863 — Commercial mortgage backed securities 545,817 2,131 (4,268 ) 547,954 — U.S. government and government agencies 3,484,257 2,188 (28,769 ) 3,510,838 — Non-U.S. government securities 1,612,754 48,764 (17,321 ) 1,581,311 — Asset backed securities 1,780,143 5,147 (8,614 ) 1,783,610 — Total 14,332,391 111,098 (106,181 ) 14,327,474 (88 ) Equity securities 504,333 88,739 (5,583 ) 421,177 — Other investments 264,989 66,946 (120 ) 198,163 — Short-term investments 1,469,042 650 (563 ) 1,468,955 — Total $ 16,570,755 $ 267,433 $ (112,447 ) $ 16,415,769 $ (88 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” (2) Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018 , the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million , compared to a net unrealized gain of $0.3 million at December 31, 2017 . (3) Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2 ). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table. The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position: Less than 12 Months 12 Months or More Total Estimated Gross Unrealized Losses Estimated Gross Unrealized Losses Estimated Gross Unrealized Losses June 30, 2018 Fixed maturities (1): Corporate bonds $ 4,541,077 $ (94,563 ) $ 236,332 $ (9,415 ) $ 4,777,409 $ (103,978 ) Mortgage backed securities 296,876 (4,618 ) 708 (124 ) 297,584 (4,742 ) Municipal bonds 814,694 (16,888 ) 109,913 (5,159 ) 924,607 (22,047 ) Commercial mortgage backed securities 351,013 (9,067 ) 56,382 (4,155 ) 407,395 (13,222 ) U.S. government and government agencies 1,957,338 (18,470 ) 41,069 (1,107 ) 1,998,407 (19,577 ) Non-U.S. government securities 1,243,055 (24,034 ) 160,434 (3,446 ) 1,403,489 (27,480 ) Asset backed securities 1,377,269 (14,096 ) 110,507 (2,478 ) 1,487,776 (16,574 ) Total 10,581,322 (181,736 ) 715,345 (25,884 ) 11,296,667 (207,620 ) Equity securities (2) Other investments — — — — — — Short-term investments 53,937 (72 ) — — 53,937 (72 ) Total $ 10,635,259 $ (181,808 ) $ 715,345 $ (25,884 ) $ 11,350,604 $ (207,692 ) December 31, 2017 Fixed maturities (1): Corporate bonds $ 2,320,716 $ (25,411 ) $ 279,082 $ (6,929 ) $ 2,599,798 $ (32,340 ) Mortgage backed securities 221,113 (1,715 ) 28,380 (846 ) 249,493 (2,561 ) Municipal bonds 1,030,389 (8,438 ) 132,469 (3,870 ) 1,162,858 (12,308 ) Commercial mortgage backed securities 225,164 (1,899 ) 57,291 (2,369 ) 282,455 (4,268 ) U.S. government and government agencies 2,646,415 (26,501 ) 111,879 (2,268 ) 2,758,294 (28,769 ) Non-U.S. government securities 1,218,514 (15,546 ) 93,530 (1,775 ) 1,312,044 (17,321 ) Asset backed securities 1,111,246 (5,915 ) 209,207 (2,699 ) 1,320,453 (8,614 ) Total 8,773,557 (85,425 ) 911,838 (20,756 ) 9,685,395 (106,181 ) Equity securities 166,562 (5,583 ) — — 166,562 (5,583 ) Other investments 15,025 (120 ) — — 15,025 (120 ) Short-term investments 109,528 (563 ) — — 109,528 (563 ) Total $ 9,064,672 $ (91,691 ) $ 911,838 $ (20,756 ) $ 9,976,510 $ (112,447 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” (2) Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2 ). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table. At June 30, 2018 , on a lot level basis, approximately 5,440 security lots out of a total of approximately 7,450 security lots were in an unrealized loss position and the largest single unrealized loss from a single lot in the Company’s fixed maturity portfolio was $1.7 million . At December 31, 2017 , on a lot level basis, approximately 3,830 security lots out of a total of approximately 7,450 security lots were in an unrealized loss position and the largest single unrealized loss from a single lot in the Company’s fixed maturity portfolio was $1.3 million . The contractual maturities of the Company’s fixed maturities are shown in the following table. Expected maturities, which are management’s best estimates, will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. June 30, 2018 December 31, 2017 Maturity Estimated Fair Value Amortized Cost Estimated Amortized Cost Due in one year or less $ 344,007 $ 343,938 $ 550,711 $ 548,771 Due after one year through five years 8,154,773 8,235,481 7,436,153 7,434,801 Due after five years through 10 years 2,703,341 2,756,097 3,369,635 3,369,750 Due after 10 years 245,953 247,730 333,791 325,526 11,448,074 11,583,246 11,690,290 11,678,848 Mortgage backed securities 488,699 491,742 316,141 317,062 Commercial mortgage backed securities 590,198 602,644 545,817 547,954 Asset backed securities 1,820,489 1,833,753 1,780,143 1,783,610 Total (1) $ 14,347,460 $ 14,511,385 $ 14,332,391 $ 14,327,474 (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” Securities Lending Agreements The Company enters into securities lending agreements with financial institutions to enhance investment income whereby it loans certain of its securities to third parties, primarily major brokerage firms, for short periods of time through a lending agent. The Company maintains legal control over the securities it lends, retains the earnings and cash flows associated with the loaned securities and receives a fee from the borrower for the temporary use of the securities. An indemnification agreement with the lending agent protects the Company in the event a borrower becomes insolvent or fails to return any of the securities on loan from the Company. The Company receives collateral in the form of cash or securities. Cash collateral primarily consists of short term investments. At June 30, 2018 , the fair value of the cash collateral received on securities lending was $28.0 million and the fair value of security collateral received was $209.0 million . At December 31, 2017 , the fair value of the cash collateral received on securities lending was $199.9 million , and the fair value of security collateral received was $276.7 million . The Company’s securities lending transactions were accounted for as secured borrowings with significant investment categories as follows: Remaining Contractual Maturity of the Agreements Overnight and Continuous Less than 30 Days 30-90 Days 90 Days or More Total June 30, 2018 U.S. government and government agencies $ 147,509 $ — $ 48,268 $ — $ 195,777 Corporate bonds 29,265 — — — 29,265 Equity securities 11,906 — — — 11,906 Total $ 188,680 $ — $ 48,268 $ — $ 236,948 Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 9 $ — Amounts related to securities lending not included in offsetting disclosure in Note 9 $ 236,948 December 31, 2017 U.S. government and government agencies $ 343,425 $ 20,309 $ 76,086 $ — $ 439,820 Corporate bonds 28,003 — — — 28,003 Equity securities 8,782 — — — 8,782 Total $ 380,210 $ 20,309 $ 76,086 $ — $ 476,605 Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 9 $ — Amounts related to securities lending not included in offsetting disclosure in Note 9 $ 476,605 Other Investments The following table summarizes the Company’s other investments, including available for sale and fair value option components: June 30, December 31, Available for sale securities: Asian and emerging markets $ — $ 135,140 Investment grade fixed income — 53,878 Credit related funds — 18,365 Other — 57,606 Total available for sale (1) — 264,989 Fair value option: Term loan investments (par value: $1,321,154 and $1,223,453) $ 1,286,305 $ 1,200,882 Mezzanine debt funds 257,750 252,160 Credit related funds 199,437 175,422 Investment grade fixed income 98,240 102,347 Asian and emerging markets 350,308 258,541 Other (2) 136,864 147,029 Total fair value option 2,328,904 2,136,381 Total $ 2,328,904 $ 2,401,370 (1) The Company reviewed the accounting treatment for three limited partnership investments which were accounted for as available for sale at December 31, 2017 during the 2018 first quarter and determined, based on reconsideration during the period of the Company’s percentage ownership, that the equity method of accounting was appropriate for such investments. (2) Includes fund investments with strategies in mortgage servicing rights, transportation, infrastructure assets and other. Certain of the Company’s other investments are in investment funds for which the Company has the option to redeem at agreed upon values as described in each investment fund’s subscription agreement. Depending on the terms of the various subscription agreements, investments in investment funds may be redeemed daily, monthly, quarterly or on other terms. Two common redemption restrictions which may impact the Company’s ability to redeem these investment funds are gates and lockups. A gate is a suspension of redemptions which may be implemented by the general partner or investment manager of the fund in order to defer, in whole or in part, the redemption request in the event the aggregate amount of redemption requests exceeds a predetermined percentage of the investment fund’s net assets which may otherwise hinder the general partner or investment manager’s ability to liquidate holdings in an orderly fashion in order to generate the cash necessary to fund extraordinarily large redemption payouts. A lockup period is the initial amount of time an investor is contractually required to hold the security before having the ability to redeem. If the investment funds are eligible to be redeemed, the time to redeem such fund can take weeks or months following the notification. Fair Value Option The following table summarizes the Company’s assets and liabilities which are accounted for using the fair value option: June 30, December 31, Fixed maturities $ 1,428,503 $ 1,642,855 Other investments 2,328,904 2,136,381 Short-term investments 217,066 297,426 Equity securities 137,138 139,575 Investments accounted for using the fair value option $ 4,111,611 $ 4,216,237 Limited Partnership Interests In the normal course of its activities, the Company invests in limited partnerships as part of its overall investment strategy. Such amounts are included in ‘investments accounted for using the equity method’ and ‘investments accounted for using the fair value option.’ The Company has determined that it is not required to consolidate these investments because it is not the primary beneficiary of the funds. The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company’s consolidated balance sheet and any unfunded commitment. The following table summarizes investments in limited partnership interests where the Company has a variable interest by balance sheet line item: June 30, December 31, Investments accounted for using the equity method (1) $ 1,292,573 $ 1,041,321 Investments accounted for using the fair value option (2) 130,471 130,471 Total $ 1,423,044 $ 1,171,792 (1) Aggregate unfunded commitments were $994.7 million at June 30, 2018 , compared to $1.02 billion at December 31, 2017 . (2) Aggregate unfunded commitments were $91.0 million at June 30, 2018 , compared to $100.4 million at December 31, 2017 . Net Investment Income The components of net investment income were derived from the following sources: June 30, 2018 2017 Three Months Ended Fixed maturities $ 115,110 $ 94,270 Equity securities 4,777 3,654 Short-term investments 4,392 2,016 Other (1) 38,168 34,076 Gross investment income 162,447 134,016 Investment expenses (26,779 ) (22,892 ) Net investment income $ 135,668 $ 111,124 Six Months Ended Fixed maturities $ 222,997 $ 188,663 Equity securities 7,345 6,297 Short-term investments 9,252 3,775 Other (1) 75,542 73,656 Gross investment income 315,136 272,391 Investment expenses (52,744 ) (43,393 ) Net investment income $ 262,392 $ 228,998 (1) Includes income distributions from investment funds, term loan investments and other items. Equity in Net Income (Loss) of Investment Funds Accounted for Using the Equity Method The Company recorded $8.5 million of equity in net income related to investment funds accounted for using the equity method in the 2018 second quarter , compared to $32.7 million for the 2017 second quarter , and $36.5 million for the six months ended June 30, 2018 , compared to $80.8 million for the 2017 period. In applying the equity method, investments are initially recorded at cost and are subsequently adjusted based on the Company’s proportionate share of the net income or loss of the funds (which include changes in the market value of the underlying securities in the funds). Such investments are generally recorded on a one to three month lag based on the availability of reports from the investment funds. Net Realized Gains (Losses) Net realized gains (losses) were as follows, excluding net impairment losses recognized in earnings: June 30, 2018 2017 Three Months Ended Available for sale securities: Gross gains on investment sales $ 18,777 $ 76,730 Gross losses on investment sales (57,711 ) (52,619 ) Change in fair value of assets and liabilities accounted for using the fair value option: Fixed maturities (22,927 ) 9,656 Other investments (254 ) 637 Equity securities 1,230 2,829 Short-term investments (136 ) 3,328 Equity securities, at fair value (1): Net realized gains (losses) on sales during the period (5,918 ) — Net unrealized gains (losses) on equity securities still held at reporting date (7,278 ) — Derivative instruments (2) (2,146 ) (4,770 ) Other (3) (248 ) (14,056 ) Net realized gains (losses) $ (76,611 ) $ 21,735 Six Months Ended Available for sale securities: Gross gains on investment sales $ 33,742 $ 145,905 Gross losses on investment sales (140,262 ) (113,981 ) Change in fair value of assets and liabilities accounted for using the fair value option: Fixed maturities (40,478 ) 30,197 Other investments (6,628 ) 17,885 Equity securities 7,898 6,374 Short-term investments (287 ) 3,332 Equity securities, at fair value (1): Net realized gains (losses) on sales during the period (11,286 ) — Net unrealized gains (losses) on equity securities still held at reporting date (14,861 ) — Derivative instruments (2) (6,109 ) (13,951 ) Other (3) (9,338 ) (19,873 ) Net realized gains (losses) $ (187,609 ) $ 55,888 (1) Pursuant to new accounting guidance (see Note 2), changes in fair value on equity securities are recorded through net income effective January 1, 2018. (2) See Note 9 for information on the Company’s derivative instruments. (3) Includes the re-measurement of contingent consideration liability amounts. Net Impairment Losses Recognized in Earnings The Company performs quarterly reviews of its available for sale investments in order to determine whether declines in fair value below the amortized cost basis were considered other-than-temporary in accordance with applicable guidance. The following table details the net impairment losses recognized in earnings by asset class: June 30, 2018 2017 Three Months Ended Fixed maturities: Mortgage backed securities $ (81 ) $ (92 ) Corporate bonds (241 ) (1,401 ) Asset backed securities (148 ) — Municipal bonds — (173 ) Total (470 ) (1,666 ) Other investments — (64 ) Net impairment losses recognized in earnings $ (470 ) $ (1,730 ) Six Months Ended Fixed maturities: Mortgage backed securities $ (123 ) $ (1,411 ) Corporate bonds (361 ) (1,402 ) Non-U.S. government securities — (198 ) Asset backed securities (148 ) — Municipal bonds — (173 ) Total (632 ) (3,184 ) Equity securities — (186 ) Other investments — (167 ) Net impairment losses recognized in earnings $ (632 ) $ (3,537 ) Net impairment losses recognized in earnings in the 2018 periods were primarily related to foreign currency fluctuations on corporate bonds and asset backed securities. The Company believes that the $0.1 million of OTTI included in accumulated other comprehensive income at June 30, 2018 on the securities which were considered by the Company to be impaired was due to market and sector-related factors ( i.e. , not credit losses). At June 30, 2018 , the Company did not intend to sell these securities, or any other securities which were in an unrealized loss position, and determined that it is more likely than not that the Company will not be required to sell such securities before recovery of their cost basis. The following table provides a roll forward of the amount related to credit losses recognized in earnings for which a portion of an OTTI was recognized in accumulated other comprehensive income: June 30, 2018 2017 Three Months Ended Balance at start of period $ 767 $ 12,537 Credit loss impairments recognized on securities not previously impaired — 31 Credit loss impairments recognized on securities previously impaired — 172 Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security — — Reductions for securities sold during the period (69 ) (8,303 ) Balance at end of period $ 698 $ 4,437 Six Months Ended Balance at start of year $ 767 $ 13,138 Credit loss impairments recognized on securities not previously impaired — 31 Credit loss impairments recognized on securities previously impaired — 195 Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security — — Reductions for securities sold during the period (69 ) (8,927 ) Balance at end of period $ 698 $ 4,437 Restricted Assets The Company is required to maintain assets on deposit, which primarily consist of fixed maturities, with various regulatory authorities to support its insurance and reinsurance operations. The Company’s insurance and reinsurance subsidiaries maintain assets in trust accounts as collateral for insurance and reinsurance transactions with affiliated companies and also have investments in segregated portfolios primarily to provide collateral or guarantees for letters of credit to third parties. See note 16, “Commitments and Contingencies,” of the notes to consolidated financial statements in the Company’s 2017 Form 10-K. The following table details the value of the Company’s restricted assets: June 30, December 31, Assets used for collateral or guarantees: Affiliated transactions $ 4,549,122 $ 4,323,726 Third party agreements 1,537,336 1,674,304 Deposits with U.S. regulatory authorities 701,026 616,987 Deposits with non-U.S. regulatory authorities 58,371 55,895 Total restricted assets $ 6,845,855 $ 6,670,912

Fair Value

Fair Value	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value	Fair Value Accounting guidance regarding fair value measurements addresses how companies should measure fair value when they are required to use a fair value measure for recognition or disclosure purposes under GAAP and provides a common definition of fair value to be used throughout GAAP. It defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly fashion between market participants at the measurement date. In addition, it establishes a three-level valuation hierarchy for the disclosure of fair value measurements. The valuation hierarchy is based upon the transparency of inputs to the valuation of an asset or liability as of the measurement date. The level in the hierarchy within which a given fair value measurement falls is determined based on the lowest level input that is significant to the measurement (Level 1 being the highest priority and Level 3 being the lowest priority). The levels in the hierarchy are defined as follows: Level 1: Inputs to the valuation methodology are observable inputs that reflect quoted prices (unadjusted) for identical assets or liabilities in active markets Level 2: Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument Level 3: Inputs to the valuation methodology are unobservable and significant to the fair value measurement Following is a description of the valuation methodologies used for securities measured at fair value, as well as the general classification of such securities pursuant to the valuation hierarchy. The Company reviews its securities measured at fair value and discusses the proper classification of such investments with investment advisers and others. The Company determines the existence of an active market based on its judgment as to whether transactions for the financial instrument occur in such market with sufficient frequency and volume to provide reliable pricing information. The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. The Company uses quoted values and other data provided by nationally recognized independent pricing sources as inputs into its process for determining fair values of its fixed maturity investments. To validate the techniques or models used by pricing sources, the Company's review process includes, but is not limited to: (i) quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated); (ii) a review of the average number of prices obtained in the pricing process and the range of resulting fair values; (iii) initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; (iv) a comparison of the fair value estimates to the Company’s knowledge of the current market; (v) a comparison of the pricing services' fair values to other pricing services' fair values for the same investments; and (vi) periodic back-testing, which includes randomly selecting purchased or sold securities and comparing the executed prices to the fair value estimates from the pricing service. A price source hierarchy was maintained in order to determine which price source would be used (i.e. , a price obtained from a pricing service with more seniority in the hierarchy will be used over a less senior one in all cases). The hierarchy prioritizes pricing services based on availability and reliability and assigns the highest priority to index providers. Based on the above review, the Company will challenge any prices for a security or portfolio which are considered not to be representative of fair value. The Company did not adjust any of the prices obtained from the independent pricing sources at June 30, 2018 . In certain circumstances, when fair values are unavailable from these independent pricing sources, quotes are obtained directly from broker-dealers who are active in the corresponding markets. Such quotes are subject to the validation procedures noted above. Of the $20.24 billion of financial assets and liabilities measured at fair value at June 30, 2018 , approximately $276.8 million , or 1.4% , were priced using non-binding broker-dealer quotes. Of the $20.92 billion of financial assets and liabilities measured at fair value at December 31, 2017 , approximately $181.5 million , or 0.9% , were priced using non-binding broker-dealer quotes. Fixed maturities The Company uses the market approach valuation technique to estimate the fair value of its fixed maturity securities, when possible. The market approach includes obtaining prices from independent pricing services, such as index providers and pricing vendors, as well as to a lesser extent quotes from broker-dealers. The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. Each source has its own proprietary method for determining the fair value of securities that are not actively traded. In general, these methods involve the use of “matrix pricing” in which the independent pricing source uses observable market inputs including, but not limited to, investment yields, credit risks and spreads, benchmarking of like securities, broker-dealer quotes, reported trades and sector groupings to determine a reasonable fair value. The following describes the significant inputs generally used to determine the fair value of the Company’s fixed maturity securities by asset class: • U.S. government and government agencies — valuations provided by independent pricing services, with all prices provided through index providers and pricing vendors. The Company determined that all U.S. Treasuries would be classified as Level 1 securities due to observed levels of trading activity, the high number of strongly correlated pricing quotes received on U.S. Treasuries and other factors. The fair values of U.S. government agency securities are generally determined using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are classified within Level 2. • Corporate bonds — valuations provided by independent pricing services, substantially all through index providers and pricing vendors with a small amount through broker-dealers. The fair values of these securities are generally determined using the spread above the risk-free yield curve. These spreads are generally obtained from the new issue market, secondary trading and from broker-dealers who trade in the relevant security market. As the significant inputs used in the pricing process for corporate bonds are observable market inputs, the fair value of these securities are classified within Level 2. A small number of securities are included in Level 3 due to a low level of transparency on the inputs used in the pricing process. • Mortgage-backed securities — valuations provided by independent pricing services, substantially all through pricing vendors and index providers with a small amount through broker-dealers. The fair values of these securities are generally determined through the use of pricing models (including Option Adjusted Spread) which use spreads to determine the expected average life of the securities. These spreads are generally obtained from the new issue market, secondary trading and from broker-dealers who trade in the relevant security market. The pricing services also review prepayment speeds and other indicators, when applicable. As the significant inputs used in the pricing process for mortgage-backed securities are observable market inputs, the fair value of these securities are classified within Level 2. A small number of securities are included in Level 3 due to a low level of transparency on the inputs used in the pricing process. • Municipal bonds — valuations provided by independent pricing services, with all prices provided through index providers and pricing vendors. The fair values of these securities are generally determined using spreads obtained from broker-dealers who trade in the relevant security market, trade prices and the new issue market. As the significant inputs used in the pricing process for municipal bonds are observable market inputs, the fair value of these securities are classified within Level 2. • Commercial mortgage-backed securities — valuations provided by independent pricing services, substantially all through index providers and pricing vendors with a small amount through broker-dealers. The fair values of these securities are generally determined through the use of pricing models which use spreads to determine the appropriate average life of the securities. These spreads are generally obtained from the new issue market, secondary trading and from broker-dealers who trade in the relevant security market. The pricing services also review prepayment speeds and other indicators, when applicable. As the significant inputs used in the pricing process for commercial mortgage-backed securities are observable market inputs, the fair value of these securities are classified within Level 2. A small number of securities are included in Level 3 due to a low level of transparency on the inputs used in the pricing process. • Non-U.S. government securities — valuations provided by independent pricing services, with all prices provided through index providers and pricing vendors. The fair values of these securities are generally based on international indices or valuation models which include daily observed yield curves, cross-currency basis index spreads and country credit spreads. As the significant inputs used in the pricing process for non-U.S. government securities are observable market inputs, the fair value of these securities are classified within Level 2. • Asset-backed securities — valuations provided by independent pricing services, substantially all through index providers and pricing vendors with a small amount through broker-dealers. The fair values of these securities are generally determined through the use of pricing models (including Option Adjusted Spread) which use spreads to determine the appropriate average life of the securities. These spreads are generally obtained from the new issue market, secondary trading and from broker-dealers who trade in the relevant security market. The pricing services also review prepayment speeds and other indicators, when applicable. As the significant inputs used in the pricing process for asset-backed securities are observable market inputs, the fair value of these securities are classified within Level 2. A small number of securities are included in Level 3 due to a low level of transparency on the inputs used in the pricing process. Equity securities The Company determined that exchange-traded equity securities would be included in Level 1 as their fair values are based on quoted market prices in active markets. Other equity securities are included in Level 2 of the valuation hierarchy. Other investments The Company determined that exchange-traded investments in mutual funds would be included in Level 1 as their fair values are based on quoted market prices in active markets. Other investments also include term loan investments for which fair values are estimated by using quoted prices of term loan investments with similar characteristics, pricing models or matrix pricing. Such investments are generally classified within Level 2. The fair values for certain of the Company’s other investments are determined using net asset values as advised by external fund managers. The net asset value is based on the fund manager’s valuation of the underlying holdings in accordance with the fund’s governing documents. In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. A small number of securities are included in Level 3 due to the lack of an available independent price source for such securities. Derivative instruments The Company’s futures contracts, foreign currency forward contracts, interest rate swaps and other derivatives trade in the over-the-counter derivative market. The Company uses the market approach valuation technique to estimate the fair value for these derivatives based on significant observable market inputs from third party pricing vendors, non-binding broker-dealer quotes and/or recent trading activity. As the significant inputs used in the pricing process for these derivative instruments are observable market inputs, the fair value of these securities are classified within Level 2. Short-term investments The Company determined that certain of its short-term investments held in highly liquid money market-type funds, Treasury bills and commercial paper would be included in Level 1 as their fair values are based on quoted market prices in active markets. The fair values of other short-term investments are generally determined using the spread above the risk-free yield curve and are classified within Level 2. Contingent consideration liabilities Contingent consideration liabilities (included in ‘other liabilities’ in the consolidated balance sheets) include amounts related to the acquisition of CMG Mortgage Insurance Company and its affiliated mortgage insurance companies and other acquisitions. Such amounts are remeasured at fair value at each balance sheet date with changes in fair value recognized in ‘net realized gains (losses).’ To determine the fair value of contingent consideration liabilities, the Company estimates future payments using an income approach based on modeled inputs which include a weighted average cost of capital. The Company determined that contingent consideration liabilities would be included within Level 3. The following table presents the Company’s financial assets and liabilities measured at fair value by level at June 30, 2018 : Estimated Fair Value Measurements Using: Estimated Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets measured at fair value (1): Available for sale securities: Fixed maturities: Corporate bonds $ 5,535,763 $ — $ 5,526,990 $ 8,773 Mortgage backed securities 488,699 — 488,352 347 Municipal bonds 1,431,256 — 1,431,256 — Commercial mortgage backed securities 590,198 — 590,169 29 U.S. government and government agencies 2,788,272 2,660,621 127,651 — Non-U.S. government securities 1,692,783 — 1,692,783 — Asset backed securities 1,820,489 — 1,820,489 — Total 14,347,460 2,660,621 11,677,690 9,149 Short-term investments 1,096,798 1,053,661 43,137 — Equity securities, at fair value 546,075 493,975 52,100 — Derivative instruments (4) 31,623 — 31,623 — Fair value option: Corporate bonds 936,840 — 925,505 11,335 Non-U.S. government bonds 132,162 — 132,162 — Mortgage backed securities 16,410 — 16,410 — Municipal bonds 7,082 — 7,082 — Commercial mortgage backed securities 1,306 — 1,306 — Asset backed securities 214,718 — 214,718 — U.S. government and government agencies 119,985 119,875 110 — Short-term investments 217,066 19,869 197,197 — Equity securities 137,138 65,400 71,738 — Other investments 1,260,645 65,665 1,136,766 58,214 Other investments measured at net asset value (2) 1,068,259 Total 4,111,611 270,809 2,702,994 69,549 Total assets measured at fair value $ 20,133,567 $ 4,479,066 $ 14,507,544 $ 78,698 Liabilities measured at fair value: Contingent consideration liabilities $ (63,930 ) $ — $ — $ (63,930 ) Securities sold but not yet purchased (3) (24,529 ) — (24,529 ) — Derivative instruments (4) (22,398 ) — (22,398 ) — Total liabilities measured at fair value $ (110,857 ) $ — $ (46,927 ) $ (63,930 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See Note 7 , “Investment Information—Securities Lending Agreements.” (2) In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (3) Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets. (4) See Note 9 , “Derivative Instruments.” The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2017 : Estimated Fair Value Measurements Using: Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets measured at fair value (1): Available for sale securities: Fixed maturities: Corporate bonds $ 4,434,439 $ — $ 4,424,979 $ 9,460 Mortgage backed securities 316,141 — 315,754 387 Municipal bonds 2,158,840 — 2,158,840 — Commercial mortgage backed securities 545,817 — 545,277 540 U.S. government and government agencies 3,484,257 3,408,902 75,355 — Non-U.S. government securities 1,612,754 — 1,612,754 — Asset backed securities 1,780,143 — 1,775,143 5,000 Total 14,332,391 3,408,902 10,908,102 15,387 Equity securities 504,333 498,182 6,151 — Short-term investments 1,469,042 1,420,732 48,310 — Other investments 76,427 74,611 1,816 — Other investments measured at net asset value (2) 188,562 Total other investments 264,989 74,611 1,816 — Derivative instruments (4) 15,747 — 15,747 — Fair value option: Corporate bonds 1,068,725 — 1,056,508 12,217 Non-U.S. government bonds 195,788 — 195,788 — Mortgage backed securities 20,491 — 20,491 — Municipal bonds 15,210 — 15,210 — Commercial mortgage backed securities 11,997 — 11,997 — Asset backed securities 99,354 — 99,354 — U.S. government and government agencies 231,290 231,019 271 — Short-term investments 297,426 40,166 257,260 — Equity securities 139,575 67,440 72,135 — Other investments 1,128,094 82,291 986,636 59,167 Other investments measured at net asset value (2) 1,008,287 Total 4,216,237 420,916 2,715,650 71,384 Total assets measured at fair value $ 20,802,739 $ 5,823,343 $ 13,695,776 $ 86,771 Liabilities measured at fair value: Contingent consideration liabilities $ (60,996 ) $ — $ — $ (60,996 ) Securities sold but not yet purchased (3) (34,375 ) — (34,375 ) — Derivative instruments (4) (20,464 ) — (20,464 ) — Total liabilities measured at fair value $ (115,835 ) $ — $ (54,839 ) $ (60,996 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See Note 7 , “Investment Information—Securities Lending Agreements.” (2) In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (3) Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets. (4) See Note 9 , “Derivative Instruments.” The following table presents a reconciliation of the beginning and ending balances for all financial assets and liabilities measured at fair value on a recurring basis using Level 3 inputs: Assets Liabilities s Available For Sale Fair Value Option Structured Securities (1) Corporate Corporate Other Investments Total Contingent Consideration Liabilities Three Months Ended June 30, 2018 Balance at beginning of period $ 5,413 $ 9,152 $ 11,872 $ 58,452 $ 84,889 $ (62,449 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3 — (537 ) 336 (198 ) (1,481 ) Included in other comprehensive income (4 ) (316 ) — — (320 ) — Purchases, issuances, sales and settlements Purchases — 393 — — 393 — Issuances — — — — — — Sales (5,003 ) — — (74 ) (5,077 ) — Settlements (33 ) (456 ) — (500 ) (989 ) — Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ 376 $ 8,773 $ 11,335 $ 58,214 $ 78,698 $ (63,930 ) Three Months Ended June 30, 2017 Balance at beginning of period $ 10,637 $ 18,601 $ — $ 25,000 $ 54,238 $ (125,544 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3,072 636 — — 3,708 (3,441 ) Included in other comprehensive income — — — — — — Purchases, issuances, sales and settlements Purchases — 4,935 — — 4,935 — Issuances — — — — — — Sales (13,640 ) (12,602 ) — — (26,242 ) — Settlements (69 ) — — — (69 ) 71,739 Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ — $ 11,570 $ — $ 25,000 $ 36,570 $ (57,246 ) Six Months Ended June 30, 2018 Balance at beginning of year $ 5,927 $ 9,460 $ 12,217 $ 59,167 $ 86,771 $ (60,996 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 4 — (612 ) (379 ) (987 ) (2,934 ) Included in other comprehensive income (8 ) (168 ) — — (176 ) — Purchases, issuances, sales and settlements Purchases — 393 — — 393 — Issuances — — — — — — Sales (5,003 ) — — (74 ) (5,077 ) — Settlements (544 ) (912 ) (270 ) (500 ) (2,226 ) — Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ 376 $ 8,773 $ 11,335 $ 58,214 $ 78,698 $ (63,930 ) Six Months Ended June 30, 2017 Balance at beginning of year $ 11,289 $ 18,344 $ — $ 25,000 $ 54,633 $ (122,350 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3,779 893 — — 4,672 (7,087 ) Included in other comprehensive income — — — — — — Purchases, issuances, sales and settlements Purchases — 4,935 — — 4,935 — Issuances — — — — — — Sales (13,640 ) (12,602 ) — — (26,242 ) — Settlements (1,428 ) — — — (1,428 ) 72,191 Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ — $ 11,570 $ — $ 25,000 $ 36,570 $ (57,246 ) (1) Includes asset backed securities, mortgage backed securities and commercial mortgage backed securities. (2) Gains or losses were included in net realized gains (losses). Financial Instruments Disclosed, But Not Carried, At Fair Value The Company uses various financial instruments in the normal course of its business. The carrying values of cash, accrued investment income, receivable for securities sold, certain other assets, payable for securities purchased and certain other liabilities approximated their fair values at June 30, 2018 , due to their respective short maturities. As these financial instruments are not actively traded, their respective fair values are classified within Level 2. At June 30, 2018 , the Company’s senior notes were carried at their cost, net of debt issuance costs, of $1.73 billion and had a fair value of $1.89 billion . At December 31, 2017 , Company’s senior notes were carried at their cost, net of debt issuance costs, of $1.73 billion and had a fair value of $2.04 billion . The fair values of the senior notes were obtained from a third party pricing service and are based on observable market inputs. As such, the fair values of the senior notes are classified within Level 2.

Derivative Instruments

Derivative Instruments	6 Months Ended
	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments The Company’s investment strategy allows for the use of derivative instruments. The Company’s derivative instruments are recorded on its consolidated balance sheets at fair value. The Company utilizes exchange traded U.S. Treasury note, Eurodollar and other futures contracts and commodity futures to manage portfolio duration or replicate investment positions in its portfolios and the Company routinely utilizes foreign currency forward contracts, currency options, index futures contracts and other derivatives as part of its total return objective. In addition, certain of the Company’s investments are managed in portfolios which incorporate the use of foreign currency forward contracts which are intended to provide an economic hedge against foreign currency movements. In addition, the Company purchases to-be-announced mortgage backed securities (“TBAs”) as part of its investment strategy. TBAs represent commitments to purchase a future issuance of agency mortgage backed securities. For the period between purchase of a TBA and issuance of the underlying security, the Company’s position is accounted for as a derivative. The Company purchases TBAs in both long and short positions to enhance investment performance and as part of its overall investment strategy. The following table summarizes information on the fair values and notional values of the Company’s derivative instruments: Estimated Fair Value Asset Derivatives Liability Derivatives Notional Value (1) June 30, 2018 Futures contracts (2) $ 556 $ (3,425 ) $ 1,276,189 Foreign currency forward contracts (2) 18,183 (7,732 ) 1,072,743 TBAs (3) 14,761 — 14,480 Other (2) 12,884 (11,241 ) 1,924,299 Total $ 46,384 $ (22,398 ) December 31, 2017 Futures contracts (2) $ 3,371 $ (1,542 ) $ 1,452,497 Foreign currency forward contracts (2) 4,478 (4,381 ) 686,941 TBAs (3) 27,184 — 27,066 Other (2) 7,898 (14,541 ) 1,457,345 Total $ 42,931 $ (20,464 ) (1) Represents the absolute notional value of all outstanding contracts, consisting of long and short positions. (2) The fair value of asset derivatives are included in ‘other assets’ and the fair value of liability derivatives are included in ‘other liabilities.’ (3) The fair value of TBAs are included in ‘fixed maturities available for sale, at fair value.’ The Company did not hold any derivatives which were designated as hedging instruments at June 30, 2018 or December 31, 2017 . The Company’s derivative instruments can be traded under master netting agreements, which establish terms that apply to all derivative transactions with a counterparty. In the event of a bankruptcy or other stipulated event of default, such agreements provide that the non-defaulting party may elect to terminate all outstanding derivative transactions, in which case all individual derivative positions (loss or gain) with a counterparty are closed out and netted and replaced with a single amount, usually referred to as the termination amount, which is expressed in a single currency. The resulting single net amount, where positive, is payable to the party “in-the-money” regardless of whether or not it is the defaulting party, unless the parties have agreed that only the non-defaulting party is entitled to receive a termination payment where the net amount is positive and is in its favor. Contractual close-out netting reduces derivatives credit exposure from gross to net exposure. The remaining derivatives included in the table above were not subject to a master netting agreement. At June 30, 2018 , asset derivatives and liability derivatives of $44.0 million and $20.1 million , respectively, were subject to a master netting agreement, compared to $40.6 million and $19.6 million , respectively, at December 31, 2017 . The remaining derivatives included in the preceding table were not subject to a master netting agreement. Realized and unrealized contract gains and losses on the Company’s derivative instruments are reflected in ‘net realized gains (losses)’ in the consolidated statements of income, as summarized in the following table: Derivatives not designated as June 30, hedging instruments: 2018 2017 Three Months Ended Net realized gains (losses): Futures contracts $ (240 ) $ (5,310 ) Foreign currency forward contracts (1,692 ) (272 ) TBAs — 86 Other (214 ) 726 Total $ (2,146 ) $ (4,770 ) Six Months Ended Net realized gains (losses): Futures contracts $ 4,790 $ 2,410 Foreign currency forward contracts (7,616 ) (12,038 ) TBAs (97 ) 21 Other (3,186 ) (4,344 ) Total $ (6,109 ) $ (13,951 )

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Investment Commitments The Company’s investment commitments, which are primarily related to agreements entered into by the Company to invest in funds and separately managed accounts when called upon, were approximately $1.63 billion at June 30, 2018 , compared to $1.70 billion at December 31, 2017 . Interest Paid Interest paid on the Company’s senior notes and other borrowings were $60.9 million for the six months ended June 30, 2018 , compared to $58.0 million for the 2017 period.

Variable Interest Entities and

Variable Interest Entities and Noncontrolling Interests	6 Months Ended
	Jun. 30, 2018
Noncontrolling Interest [Abstract]
Variable Interest Entity and Noncontrolling Interests	Variable Interest Entities and Noncontrolling Interests A variable interest entity (“VIE”) refers to an entity that has characteristics such as (i) insufficient equity at risk to allow the entity to finance its activities without additional financial support or (ii) instances where the equity investors, as a group, do not have characteristics of a controlling financial interest. The primary beneficiary of a VIE is defined as the variable interest holder that is determined to have the controlling financial interest as a result of having both (i) the power to direct the activities of a VIE that most significantly impact the economic performance of the VIE and (ii) the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. If a company is determined to be the primary beneficiary, it is required to consolidate the VIE in its financial statements. Watford Holdings Ltd. In March 2014, the Company invested $100.0 million and acquired approximately 11% of Watford Holdings Ltd.’s common equity and a warrant to purchase additional common equity. Watford Holdings Ltd. is the parent of Watford Re Ltd., a multi-line Bermuda reinsurance company (together with Watford Holdings Ltd., “Watford Re”). Watford Re is considered a VIE and the Company concluded that it is the primary beneficiary of Watford Re. As such, the results of Watford Re are included in the Company’s consolidated financial statements. The Company does not guarantee or provide credit support for Watford Re, and the Company’s financial exposure to Watford Re is limited to its investment in Watford Re’s common and preferred shares and counterparty credit risk (mitigated by collateral) arising from reinsurance transactions. The following table provides the carrying amount and balance sheet caption in which the assets and liabilities of Watford Re are reported: June 30, December 31, 2018 2017 Assets Investments accounted for using the fair value option $ 2,385,316 $ 2,426,066 Fixed maturities available for sale, at fair value 279,177 — Equity securities, at fair value 63,010 — Cash 45,644 54,503 Accrued investment income 18,108 18,261 Premiums receivable 232,842 177,492 Reinsurance recoverable on unpaid and paid losses and LAE 54,299 42,777 Ceded unearned premiums 55,853 24,762 Deferred acquisition costs 88,752 85,961 Receivable for securities sold 31,387 36,374 Goodwill and intangible assets 7,650 7,650 Other assets 75,478 140,808 Total assets of consolidated VIE $ 3,337,516 $ 3,014,654 Liabilities Reserves for losses and loss adjustment expenses $ 899,395 $ 798,262 Unearned premiums 384,537 330,644 Reinsurance balances payable 41,185 18,424 Revolving credit agreement borrowings 447,289 441,132 Payable for securities purchased 132,164 42,501 Other liabilities 244,320 215,186 Total liabilities of consolidated VIE $ 2,148,890 $ 1,846,149 Redeemable noncontrolling interests $ 220,805 $ 220,622 For the six months ended June 30, 2018 , Watford Re generated $92.4 million of cash provided by operating activities, $168.0 million of cash used for investing activities and $2.1 million of cash used for financing activities, compared to $134.4 million of cash provided by operating activities, $63.5 million of cash used for investing activities and $76.4 million of cash used for financing activities for the six months ended June 30, 2017 . Non-redeemable noncontrolling interests The Company accounts for the portion of Watford Re’s common equity attributable to third party investors in the shareholders’ equity section of its consolidated balance sheets. The noncontrolling ownership in Watford Re’s common shares was approximately 89% at June 30, 2018 . The portion of Watford Re’s income or loss attributable to third party investors is recorded in the consolidated statements of income in ‘net (income) loss attributable to noncontrolling interests.’ The following table sets forth activity in the non-redeemable noncontrolling interests: June 30, 2018 2017 Three Months Ended Balance, beginning of period $ 854,112 $ 868,186 Amounts attributable to noncontrolling interests 8,116 9,346 Other comprehensive income attributable to noncontrolling interests (1,075 ) (76 ) Balance, end of period $ 861,153 $ 877,456 Six Months Ended Balance, beginning of year $ 843,411 $ 851,854 Amounts attributable to noncontrolling interests 19,492 25,670 Other comprehensive income attributable to noncontrolling interests (1,750 ) (68 ) Balance, end of period $ 861,153 $ 877,456 Redeemable noncontrolling interests The Company accounts for redeemable noncontrolling interests in the mezzanine section of its consolidated balance sheets in accordance with applicable accounting guidance. Such redeemable noncontrolling interests relate to the 9,065,200 cumulative redeemable preference shares (“Watford Preference Shares”) issued in March 2014 with a par value of $0.01 per share and a liquidation preference of $25.00 per share. Preferred dividends, including the accretion of the discount and issuance costs, are included in ‘net (income) loss attributable to noncontrolling interests’ in the Company’s consolidated statements of income. The following table sets forth activity in the redeemable non-controlling interests: June 30, 2018 2017 Three Months Ended Balance, beginning of period $ 206,013 $ 205,644 Accretion of preference share issuance costs 92 92 Balance, end of period $ 206,105 $ 205,736 Six Months Ended Balance, beginning of year $ 205,922 $ 205,553 Accretion of preference share issuance costs 183 183 Balance, end of period $ 206,105 $ 205,736 The portion of Watford Re’s income or loss attributable to third party investors, recorded in the Company’s consolidated statements of income in ‘net (income) loss attributable to noncontrolling interests,’ are summarized in the table below: June 30, 2018 2017 Three Months Ended Amounts attributable to non-redeemable noncontrolling interests $ (8,116 ) $ (9,346 ) Dividends attributable to redeemable noncontrolling interests (4,585 ) (4,586 ) Net (income) loss attributable to noncontrolling interests $ (12,701 ) $ (13,932 ) Six Months Ended Amounts attributable to non-redeemable noncontrolling interests $ (19,492 ) $ (25,670 ) Dividends attributable to redeemable noncontrolling interests (9,170 ) (9,170 ) Net (income) loss attributable to noncontrolling interests $ (28,662 ) $ (34,840 ) Bellemeade Re The Company has entered into various aggregate excess of loss reinsurance agreements with Bellemeade Re I Ltd. (July 2015), with Bellemeade Re II Ltd. (May 2016), with Bellemeade 2017-1 Ltd. (October 2017) and with Bellemeade 2018-1 Ltd. (April 2018) (the “Bellemeade Agreements”), special purpose reinsurance companies domiciled in Bermuda. At the time the Bellemeade Agreements were entered into, the applicability of the accounting guidance that addresses VIEs was evaluated. As a result of the evaluation of the Bellemeade Agreements, the Company concluded that Bellemeade Re I Ltd., Bellemeade Re II Ltd., Bellemeade 2017-1 Ltd. and Bellemeade 2018-1 Ltd. are VIEs. However, given that the ceding insurers do not have the unilateral power to direct those activities that are significant to the economic performance of Bellemeade Re I Ltd., Bellemeade Re II Ltd., Bellemeade 2017-1 Ltd. and Bellemeade 2018-1 Ltd., the Company does not consolidate such companies in its consolidated financial statements. The following table presents total assets of the Bellemeade entities, as well as the Company’s maximum exposure to loss associated with these VIEs as of June 30, 2018 and December 31, 2017: Maximum Exposure to Loss Total VIE Assets On-Balance Sheet Off-Balance Sheet Total June 30, 2018 Bellemeade Re I Ltd. $ 69,079 $ 263 $ 469 $ 732 Bellemeade Re II Ltd. 55,388 14 110 124 Bellemeade 2017-1 Ltd. 329,030 815 1,759 2,574 Bellemeade 2018-1 Ltd. 374,460 1,242 2,050 3,292 Total $ 827,957 $ 2,334 $ 4,388 $ 6,722 December 31, 2017 Bellemeade Re I Ltd. $ 92,390 $ 471 $ 832 $ 1,303 Bellemeade Re II Ltd. 135,201 20 527 547 Bellemeade 2017-1 Ltd. 347,139 391 1,867 2,258 Total $ 574,730 $ 882 $ 3,226 $ 4,108

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	6 Months Ended
	Jun. 30, 2018
Comprehensive Income Note Disclosure [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) The following tables present details about amounts reclassified from accumulated other comprehensive income and the tax effects allocated to each component of other comprehensive income (loss): Amounts Reclassified from AOCI Consolidated Statement of Income Three Months Ended Six Months Ended Details About Line Item That Includes June 30, June 30, AOCI Components Reclassification 2018 2017 2018 2017 Unrealized appreciation on available-for-sale investments Net realized gains (losses) $ (38,935 ) $ 24,111 $ (106,521 ) $ 31,924 Other-than-temporary impairment losses (470 ) (1,730 ) (632 ) (3,537 ) Total before tax (39,405 ) 22,381 (107,153 ) 28,387 Income tax (expense) benefit 2,762 (5,157 ) 8,049 (6,119 ) Net of tax $ (36,643 ) $ 17,224 $ (99,104 ) $ 22,268 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Three Months Ended June 30, 2018 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ (88,034 ) $ (2,763 ) $ (85,271 ) Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income (39,405 ) (2,762 ) (36,643 ) Foreign currency translation adjustments (12,701 ) (106 ) (12,595 ) Other comprehensive income (loss) $ (61,330 ) $ (107 ) $ (61,223 ) Three Months Ended June 30, 2017 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ 108,011 $ 15,042 $ 92,969 Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income 22,381 5,157 17,224 Foreign currency translation adjustments 18,509 212 18,297 Other comprehensive income (loss) $ 104,139 $ 10,097 $ 94,042 Six Months Ended June 30, 2018 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ (277,977 ) $ (26,029 ) $ (251,948 ) Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income (107,153 ) (8,049 ) (99,104 ) Foreign currency translation adjustments (11,269 ) 44 (11,313 ) Other comprehensive income (loss) $ (182,093 ) $ (17,936 ) $ (164,157 ) Six Months Ended June 30, 2017 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ 219,483 $ 25,722 $ 193,761 Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income 28,387 6,119 22,268 Foreign currency translation adjustments 21,674 253 21,421 Other comprehensive income (loss) $ 212,770 $ 19,856 $ 192,914

Guarantor Financial Information

Guarantor Financial Information	6 Months Ended
	Jun. 30, 2018
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Guarantor Financial Information	Guarantor Financial Information The following tables present condensed financial information for Arch Capital, Arch-U.S., a 100% owned subsidiary of Arch Capital, and Arch Capital’s other subsidiaries. June 30, 2018 Condensed Consolidating Balance Sheet Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Assets Total investments $ 134 $ 198,122 $ 21,353,862 $ (14,700 ) $ 21,537,418 Cash 7,033 27,513 492,082 — 526,628 Investments in subsidiaries 9,455,746 4,033,568 — (13,489,314 ) — Due from subsidiaries and affiliates — 2 1,849,468 (1,849,470 ) — Premiums receivable — — 1,897,517 (546,207 ) 1,351,310 Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses — — 8,441,528 (5,714,225 ) 2,727,303 Contractholder receivables — — 2,044,322 — 2,044,322 Ceded unearned premiums — — 1,821,539 (806,876 ) 1,014,663 Deferred acquisition costs — — 633,454 (63,637 ) 569,817 Goodwill and intangible assets — — 593,008 — 593,008 Other assets 12,978 56,042 1,555,017 (135,386 ) 1,488,651 Total assets $ 9,475,891 $ 4,315,247 $ 40,681,797 $ (22,619,815 ) $ 31,853,120 Liabilities Reserve for losses and loss adjustment expenses $ — $ — $ 16,855,050 $ (5,430,713 ) $ 11,424,337 Unearned premiums — — 4,640,416 (806,876 ) 3,833,540 Reinsurance balances payable — — 957,289 (546,207 ) 411,082 Contractholder payables — — 2,044,322 — 2,044,322 Collateral held for insured obligations — — 257,396 257,396 Senior notes 297,101 494,672 941,438 — 1,733,211 Revolving credit agreement borrowings — — 572,289 — 572,289 Due to subsidiaries and affiliates 1,824 536,747 1,310,899 (1,849,470 ) — Other liabilities 13,211 29,304 1,785,950 (482,535 ) 1,345,930 Total liabilities 312,136 1,060,723 29,365,049 (9,115,801 ) 21,622,107 Redeemable noncontrolling interests — — 220,805 (14,700 ) 206,105 Shareholders’ Equity Total shareholders’ equity available to Arch 9,163,755 3,254,524 10,234,790 (13,489,314 ) 9,163,755 Non-redeemable noncontrolling interests — — 861,153 — 861,153 Total shareholders’ equity 9,163,755 3,254,524 11,095,943 (13,489,314 ) 10,024,908 Total liabilities, noncontrolling interests and shareholders’ equity $ 9,475,891 $ 4,315,247 $ 40,681,797 $ (22,619,815 ) $ 31,853,120 December 31, 2017 Condensed Consolidating Balance Sheet Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Assets Total investments $ 96,540 $ 46,281 $ 21,711,891 $ (14,700 ) $ 21,840,012 Cash 9,997 30,380 565,822 — 606,199 Investments in subsidiaries 9,396,621 4,097,765 — (13,494,386 ) — Due from subsidiaries and affiliates 394 — 1,828,864 (1,829,258 ) — Premiums receivable — — 2,967,701 (1,832,452 ) 1,135,249 Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses — — 8,442,192 (5,902,049 ) 2,540,143 Contractholder receivables — — 1,978,414 — 1,978,414 Ceded unearned premiums — — 2,165,789 (1,239,178 ) 926,611 Deferred acquisition costs — — 693,053 (157,229 ) 535,824 Goodwill and intangible assets — — 652,611 — 652,611 Other assets 13,176 49,585 1,860,505 (86,671 ) 1,836,595 Total assets $ 9,516,728 $ 4,224,011 $ 42,866,842 $ (24,555,923 ) $ 32,051,658 Liabilities Reserve for losses and loss adjustment expenses $ — $ — $ 17,236,401 $ (5,852,609 ) $ 11,383,792 Unearned premiums — — 4,861,491 (1,239,177 ) 3,622,314 Reinsurance balances payable — — 2,155,947 (1,832,451 ) 323,496 Contractholder payables — — 1,978,414 — 1,978,414 Collateral held for insured obligations — — 240,183 — 240,183 Senior notes 297,053 494,621 941,210 — 1,732,884 Revolving credit agreement borrowings — — 816,132 — 816,132 Due to subsidiaries and affiliates 235 536,919 1,292,104 (1,829,258 ) — Other liabilities 22,838 29,317 1,949,696 (293,343 ) 1,708,508 Total liabilities 320,126 1,060,857 31,471,578 (11,046,838 ) 21,805,723 Redeemable noncontrolling interests — — 220,622 (14,700 ) 205,922 Shareholders’ Equity Total shareholders’ equity available to Arch 9,196,602 3,163,154 10,331,231 (13,494,385 ) 9,196,602 Non-redeemable noncontrolling interests — — 843,411 — 843,411 Total shareholders’ equity 9,196,602 3,163,154 11,174,642 (13,494,385 ) 10,040,013 Total liabilities, noncontrolling interests and shareholders’ equity $ 9,516,728 $ 4,224,011 $ 42,866,842 $ (24,555,923 ) $ 32,051,658 Three Months Ended June 30, 2018 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 1,336,763 $ — $ 1,336,763 Net investment income 15 560 157,532 (22,439 ) 135,668 Net realized gains (losses) — — (76,611 ) — (76,611 ) Net impairment losses recognized in earnings — — (470 ) — (470 ) Other underwriting income — — 3,874 — 3,874 Equity in net income (loss) of investment funds accounted for using the equity method — — 8,472 — 8,472 Other income (loss) 2,339 — 774 — 3,113 Total revenues 2,354 560 1,430,334 (22,439 ) 1,410,809 Expenses Losses and loss adjustment expenses — — 726,153 — 726,153 Acquisition expenses — — 202,838 — 202,838 Other operating expenses — — 176,181 — 176,181 Corporate expenses 16,642 470 5,400 — 22,512 Amortization of intangible assets — — 26,472 — 26,472 Interest expense 5,537 12,013 34,911 (22,117 ) 30,344 Net foreign exchange (gains) losses — — (43,357 ) (10,349 ) (53,706 ) Total expenses 22,179 12,483 1,128,598 (32,466 ) 1,130,794 Income (loss) before income taxes (19,825 ) (11,923 ) 301,736 10,027 280,015 Income tax (expense) benefit — 2,477 (26,145 ) — (23,668 ) Income (loss) before equity in net income of subsidiaries (19,825 ) (9,446 ) 275,591 10,027 256,347 Equity in net income of subsidiaries 263,471 86,727 — (350,198 ) — Net income 243,646 77,281 275,591 (340,171 ) 256,347 Net (income) loss attributable to noncontrolling interests — — (13,023 ) 322 (12,701 ) Net income available to Arch 243,646 77,281 262,568 (339,849 ) 243,646 Preferred dividends (10,403 ) — — — (10,403 ) Net income available to Arch common shareholders $ 233,243 $ 77,281 $ 262,568 $ (339,849 ) $ 233,243 Comprehensive income available to Arch $ 183,500 $ 70,066 $ 212,802 $ (282,868 ) $ 183,500 Three Months Ended June 30, 2017 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 1,240,874 $ — $ 1,240,874 Net investment income 1 184 133,627 (22,688 ) 111,124 Net realized gains (losses) — — 21,735 — 21,735 Net impairment losses recognized in earnings — — (1,730 ) — (1,730 ) Other underwriting income — — 4,822 — 4,822 Equity in net income (loss) of investment funds accounted for using the equity method — — 32,706 — 32,706 Other income (loss) (437 ) — (1,557 ) — (1,994 ) Total revenues (436 ) 184 1,430,477 (22,688 ) 1,407,537 Expenses Losses and loss adjustment expenses — — 689,860 — 689,860 Acquisition expenses — — 190,436 — 190,436 Other operating expenses — — 169,981 — 169,981 Corporate expenses 21,816 1,309 1,751 — 24,876 Amortization of intangible assets — — 30,824 — 30,824 Interest expense 6,075 11,989 33,050 (22,365 ) 28,749 Net foreign exchange (gains) losses — — 29,843 9,700 39,543 Total expenses 27,891 13,298 1,145,745 (12,665 ) 1,174,269 Income (loss) before income taxes (28,327 ) (13,114 ) 284,732 (10,023 ) 233,268 Income tax (expense) benefit — 4,069 (38,238 ) — (34,169 ) Income (loss) before equity in net income of subsidiaries (28,327 ) (9,045 ) 246,494 (10,023 ) 199,099 Equity in net income of subsidiaries 213,494 86,156 — (299,650 ) — Net income 185,167 77,111 246,494 (309,673 ) 199,099 Net (income) loss attributable to noncontrolling interests — — (14,254 ) 322 (13,932 ) Net income available to Arch 185,167 77,111 232,240 (309,351 ) 185,167 Preferred dividends (11,349 ) — — — (11,349 ) Net income available to Arch common shareholders $ 173,818 $ 77,111 $ 232,240 $ (309,351 ) $ 173,818 Comprehensive income available to Arch $ 279,285 $ 105,302 $ 475,747 $ (581,049 ) $ 279,285 Six Months Ended June 30, 2018 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 2,571,662 $ — $ 2,571,662 Net investment income 35 818 306,299 (44,760 ) 262,392 Net realized gains (losses) 29 (7 ) (187,631 ) — (187,609 ) Net impairment losses recognized in earnings — — (632 ) — (632 ) Other underwriting income — — 9,223 — 9,223 Equity in net income (loss) of investment funds accounted for using the equity method — — 36,541 — 36,541 Other income (loss) 2,261 — 926 — 3,187 Total revenues 2,325 811 2,736,388 (44,760 ) 2,694,764 Expenses Losses and loss adjustment expenses — — 1,363,013 — 1,363,013 Acquisition expenses — — 394,214 — 394,214 Other operating expenses — — 351,196 — 351,196 Corporate expenses 32,811 759 4,254 — 37,824 Amortization of intangible assets — — 53,208 — 53,208 Interest expense 11,073 23,939 70,083 (44,115 ) 60,980 Net foreign exchange (gains) losses 29 — (26,921 ) (7,093 ) (33,985 ) Total expenses 43,913 24,698 2,209,047 (51,208 ) 2,226,450 Income (loss) before income taxes (41,588 ) (23,887 ) 527,341 6,448 468,314 Income tax (expense) benefit — 5,428 (51,011 ) — (45,583 ) Income (loss) before equity in net income of subsidiaries (41,588 ) (18,459 ) 476,330 6,448 422,731 Equity in net income of subsidiaries 435,657 173,147 — (608,804 ) — Net income 394,069 154,688 476,330 (602,356 ) 422,731 Net (income) loss attributable to noncontrolling interests — — (29,307 ) 645 (28,662 ) Net income available to Arch 394,069 154,688 447,023 (601,711 ) 394,069 Preferred dividends (20,840 ) — — — (20,840 ) Loss on redemption of preferred shares (2,710 ) — — — (2,710 ) Net income available to Arch common shareholders $ 370,519 $ 154,688 $ 447,023 $ (601,711 ) $ 370,519 Comprehensive income available to Arch $ 231,662 $ 76,603 $ 291,883 $ (368,486 ) $ 231,662 Six Months Ended June 30, 2017 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 2,357,891 $ — $ 2,357,891 Net investment income 6 1,000 271,608 (43,616 ) 228,998 Net realized gains (losses) — — 55,888 — 55,888 Net impairment losses recognized in earnings — — (3,537 ) — (3,537 ) Other underwriting income — — 9,455 — 9,455 Equity in net income (loss) of investment funds accounted for using the equity method — — 80,794 — 80,794 Other income (loss) (266 ) — (2,510 ) — (2,776 ) Total revenues (260 ) 1,000 2,769,589 (43,616 ) 2,726,713 Expenses Losses and loss adjustment expenses — — 1,242,430 — 1,242,430 Acquisition expenses — — 372,725 — 372,725 Other operating expenses — — 344,700 — 344,700 Corporate expenses 39,063 3,317 10,288 — 52,668 Amortization of intangible assets — — 62,118 — 62,118 Interest expense 12,090 23,919 64,386 (42,970 ) 57,425 Net foreign exchange (gains) losses — — 45,191 13,756 58,947 Total expenses 51,153 27,236 2,141,838 (29,214 ) 2,191,013 Income (loss) before income taxes (51,413 ) (26,236 ) 627,751 (14,402 ) 535,700 Income tax (expense) benefit — 8,942 (71,508 ) — (62,566 ) Income (loss) before equity in net income of subsidiaries (51,413 ) (17,294 ) 556,243 (14,402 ) 473,134 Equity in net income of subsidiaries 489,707 163,529 — (653,236 ) — Net income 438,294 146,235 556,243 (667,638 ) 473,134 Net (income) loss attributable to noncontrolling interests — — (35,485 ) 645 (34,840 ) Net income available to Arch 438,294 146,235 520,758 (666,993 ) 438,294 Preferred dividends (22,567 ) — — — (22,567 ) Net income available to Arch common shareholders $ 415,727 $ 146,235 $ 520,758 $ (666,993 ) $ 415,727 Comprehensive income available to Arch $ 631,276 $ 193,083 $ 699,920 $ (893,003 ) $ 631,276 Six Months Ended June 30, 2018 Condensed Consolidating Statement of Cash Flows Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Operating Activities Net Cash Provided By (Used For) Operating Activities $ 201,487 $ 149,576 $ 1,010,703 $ (864,880 ) $ 496,886 Investing Activities Purchases of fixed maturity investments — (125,440 ) (17,347,846 ) 605,716 (16,867,570 ) Purchases of equity securities — — (679,663 ) — (679,663 ) Purchases of other investments — — (1,017,147 ) — (1,017,147 ) Proceeds from the sales of fixed maturity investments — 33,793 16,662,466 (605,716 ) 16,090,543 Proceeds from the sales of equity securities — — 622,068 — 622,068 Proceeds from the sales, redemptions and maturities of other investments — — 773,298 — 773,298 Proceeds from redemptions and maturities of fixed maturity investments — — 511,448 — 511,448 Net settlements of derivative instruments — — 4,498 — 4,498 Net (purchases) sales of short-term investments 96,446 (59,798 ) 415,253 — 451,901 Change in cash collateral related to securities lending — — 176,304 — 176,304 Contributions to subsidiaries — (1,000 ) (22,595 ) 23,595 — Purchases of fixed assets (71 ) — (13,171 ) — (13,242 ) Other (4 ) — 49,965 — 49,961 Net Cash Provided By (Used For) Investing Activities 96,371 (152,445 ) 134,878 23,595 102,399 Financing Activities Redemption of preferred shares (92,555 ) — — — (92,555 ) Purchases of common shares under share repurchase program (173,575 ) — — — (173,575 ) Proceeds from common shares issued, net (13,851 ) — 23,595 (23,595 ) (13,851 ) Proceeds from borrowings — — 130,579 — 130,579 Repayments of borrowings — — (373,000 ) — (373,000 ) Change in cash collateral related to securities lending — — (176,304 ) — (176,304 ) Dividends paid to redeemable noncontrolling interests — — (9,632 ) 638 (8,994 ) Dividends paid to parent (1) — — (864,242 ) 864,242 — Other — — (4,489 ) — (4,489 ) Preferred dividends paid (20,840 ) — — — (20,840 ) Net Cash Provided By (Used For) Financing Activities (300,821 ) — (1,273,493 ) 841,285 (733,029 ) Effects of exchange rates changes on foreign currency cash and restricted cash — — (10,431 ) — (10,431 ) Increase (decrease) in cash and restricted cash (2,963 ) (2,869 ) (138,343 ) — (144,175 ) Cash and restricted cash, beginning of year 10,048 30,380 686,856 — 727,284 Cash and restricted cash, end of period $ 7,085 $ 27,511 $ 548,513 $ — $ 583,109 (1) Dividends received by parent are included in net cash provided by (used for) operating activities. Six Months Ended June 30, 2017 Condensed Consolidating Statement of Cash Flows Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Operating Activities Net Cash Provided By (Used For) Operating Activities $ 7,483 $ 27,853 $ 956,437 $ (458,238 ) $ 533,535 Investing Activities Purchases of fixed maturity investments — — (19,899,326 ) — (19,899,326 ) Purchases of equity securities — — (400,155 ) — (400,155 ) Purchases of other investments — — (883,704 ) — (883,704 ) Proceeds from the sales of fixed maturity investments — — 19,611,680 — 19,611,680 Proceeds from the sales of equity securities — — 473,064 — 473,064 Proceeds from the sales, redemptions and maturities of other investments — — 614,494 — 614,494 Proceeds from redemptions and maturities of fixed maturity investments — — 447,941 — 447,941 Net settlements of derivative instruments — — (5,984 ) — (5,984 ) Net (purchases) sales of short-term investments 2,354 (27,896 ) (419,661 ) — (445,203 ) Change in cash collateral related to securities lending — — 175,693 — 175,693 Contributions to subsidiaries 20,641 (72,900 ) (342,950 ) 395,209 — Issuance of intercompany loans — — (47,000 ) 47,000 — Repayment of intercompany loans — 47,000 — (47,000 ) — Purchases of fixed assets (18 ) (10 ) (11,075 ) — (11,103 ) Other — — 54,129 (20,641 ) 33,488 Net Cash Provided By (Used For) Investing Activities 22,977 (53,806 ) (632,854 ) 374,568 (289,115 ) Financing Activities Proceeds from common shares issued, net (6,838 ) — 395,209 (395,209 ) (6,838 ) Repayments of borrowings — — (72,000 ) — (72,000 ) Change in cash collateral related to securities lending — — (175,693 ) — (175,693 ) Dividends paid to redeemable noncontrolling interests — — (9,632 ) 638 (8,994 ) Dividends paid to parent (1) — — (457,600 ) 457,600 — Other — — (62,339 ) 20,641 (41,698 ) Preferred dividends paid (22,567 ) — — — (22,567 ) Net Cash Provided By (Used For) Financing Activities (29,405 ) — (382,055 ) 83,670 (327,790 ) Effects of exchange rates changes on foreign currency cash and restricted cash — — 9,616 — 9,616 Increase (decrease) in cash and restricted cash 1,055 (25,953 ) (48,856 ) — (73,754 ) Cash and restricted cash, beginning of year 1,738 71,955 895,876 — 969,569 Cash and restricted cash, end of period $ 2,793 $ 46,002 $ 847,020 $ — $ 895,815 (1) Dividends received by parent are included in net cash provided by (used for) operating activities.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company’s income tax provision on income before income taxes resulted in an expense of 9.7% for the six months ended June 30, 2018 , compared to an expense of 11.7% for the 2017 period. The Company’s effective tax rate, which is based upon the expected annual effective tax rate, may fluctuate from period to period based on the relative mix of income or loss reported by jurisdiction and the varying tax rates in each jurisdiction. For interim reporting purposes, the Company has calculated its effective tax rate for the full year of 2018 by treating any excess tax benefits that arise from the accounting for stock based compensation as a discrete item. As such, this amount is not included when projecting the Company’s full year effective tax rate but rather is accounted for at the U.S. Federal statutory rate of 21% after applying the projected full year effective tax rate to actual results before the discrete item. The impact of the discrete item resulted in a benefit of 0.6% for the six months ended June 30, 2018 . On December 22, 2017, Staff Accounting Bulletin No. 118 (“SAB 118”) was issued to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared or analyzed in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act of 2017 (“Tax Cuts Act”). Pursuant to the guidance within SAB 118, the Company’s remeasurement of its deferred taxes at December 31, 2017 included certain provisional effects associated with enactment of the Tax Cuts Act for which measurement could be reasonably estimated. Provisional amounts may be adjusted in 2018 during the measurement period in accordance with SAB 118 when additional information is obtained. Additional information that may affect the provisional amounts would include, completion of the Company’s U.S. subsidiaries’ 2017 tax return filings, and potential future guidance from the IRS with respect to the transitional adjustment pertaining to loss reserve discounting as well as the utilization of alternative minimum tax credits. The Company’s income tax provision six months ended June 30, 2018 does not include any adjustments to the provisional effects recorded at December 31, 2017. The Company had a net deferred tax asset of $9.7 million at June 30, 2018 , compared to $39.6 million at December 31, 2017 . In addition, the Company recovered $46.5 million and paid $3.9 million of income taxes for the six months ended June 30, 2018 and 2017 , respectively.

Legal Proceedings

Legal Proceedings	6 Months Ended
	Jun. 30, 2018
Disclosure Legal Proceedings [Abstract]
Legal Proceedings	Legal Proceedings The Company, in common with the insurance industry in general, is subject to litigation and arbitration in the normal course of its business. As of June 30, 2018 , the Company was not a party to any litigation or arbitration which is expected by management to have a material adverse effect on the Company’s results of operations and financial condition and liquidity.

General (Policies)

General (Policies)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The interim consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). All significant intercompany transactions and balances have been eliminated in consolidation.
Use of estimates	The preparation of 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 the reported amounts of revenues and expenses during the reporting period. Actual results could differ materially from those estimates and assumptions. In the opinion of management, the accompanying unaudited interim consolidated financial statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. The results of any interim period are not necessarily indicative of the results for a full year or any future periods.

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Schedule of computation of basic and diluted earnings per common share	The following table sets forth the computation of basic and diluted earnings per common share: Three Months Ended Six Months Ended June 30, June 30, 2018 2017 2018 2017 Numerator: Net income $ 256,347 $ 199,099 $ 422,731 $ 473,134 Amounts attributable to noncontrolling interests (12,701 ) (13,932 ) (28,662 ) (34,840 ) Net income available to Arch 243,646 185,167 394,069 438,294 Preferred dividends (10,403 ) (11,349 ) (20,840 ) (22,567 ) Loss on redemption of preferred shares — — (2,710 ) — Net income available to Arch common shareholders $ 233,243 $ 173,818 $ 370,519 $ 415,727 Denominator: Weighted average common shares outstanding 404,800,421 369,027,702 399,485,135 366,436,407 Series D preferred shares (1) — 34,432,290 6,677,373 36,349,722 Weighted average common shares and common share equivalents outstanding — basic 404,800,421 403,459,992 406,162,508 402,786,129 Effect of dilutive common share equivalents: Nonvested restricted shares 1,575,749 4,448,889 1,837,356 4,670,889 Stock options (2) 6,735,035 9,825,057 7,460,892 9,964,878 Weighted average common shares and common share equivalents outstanding — diluted 413,111,205 417,733,938 415,460,756 417,421,896 Earnings per common share: Basic $ 0.58 $ 0.43 $ 0.91 $ 1.03 Diluted $ 0.56 $ 0.42 $ 0.89 $ 1.00 (1) Such shares are convertible non-voting common equivalent preferred shares issued in connection with the UGC acquisition. See Note 3 . (2) Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For the 2018 second quarter and 2017 second quarter , the number of stock options excluded were 5,350,733 and 1,499,997 , respectively. For the six months ended June 30, 2018 and 2017 , the number of stock options excluded were 5,372,789 and 2,292,228 , respectively.

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Analysis of underwriting income or loss by segment and reconciliation to net income available to common shareholders	The following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income or loss to net income available to Arch common shareholders: Three Months Ended June 30, 2018 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 769,372 $ 490,327 $ 330,990 $ 1,591,202 $ 175,175 $ 1,696,544 Premiums ceded (245,265 ) (136,247 ) (50,867 ) (432,892 ) (34,589 ) (397,648 ) Net premiums written 524,107 354,080 280,123 1,158,310 140,586 1,298,896 Change in unearned premiums 22,342 (13,762 ) 10,355 18,935 18,932 37,867 Net premiums earned 546,449 340,318 290,478 1,177,245 159,518 1,336,763 Other underwriting income (loss) — (129 ) 3,315 3,186 688 3,874 Losses and loss adjustment expenses (357,465 ) (229,956 ) (21,591 ) (609,012 ) (117,141 ) (726,153 ) Acquisition expenses (90,670 ) (50,142 ) (27,737 ) (168,549 ) (34,289 ) (202,838 ) Other operating expenses (92,680 ) (35,678 ) (38,729 ) (167,087 ) (9,094 ) (176,181 ) Underwriting income (loss) $ 5,634 $ 24,413 $ 205,736 235,783 (318 ) 235,465 Net investment income 107,761 27,907 135,668 Net realized gains (losses) (59,545 ) (17,066 ) (76,611 ) Net impairment losses recognized in earnings (470 ) — (470 ) Equity in net income (loss) of investment funds accounted for using the equity method 8,472 — 8,472 Other income (loss) 3,113 — 3,113 Corporate expenses (2) (15,604 ) — (15,604 ) UGC transaction costs and other (2) (6,908 ) — (6,908 ) Amortization of intangible assets (26,472 ) — (26,472 ) Interest expense (26,058 ) (4,286 ) (30,344 ) Net foreign exchange gains (losses) 46,211 7,495 53,706 Income before income taxes 266,283 13,732 280,015 Income tax expense (23,644 ) (24 ) (23,668 ) Net income 242,639 13,708 256,347 Dividends attributable to redeemable noncontrolling interests — (4,585 ) (4,585 ) Amounts attributable to nonredeemable noncontrolling interests — (8,116 ) (8,116 ) Net income available to Arch 242,639 1,007 243,646 Preferred dividends (10,403 ) — (10,403 ) Net income available to Arch common shareholders $ 232,236 $ 1,007 $ 233,243 Underwriting Ratios Loss ratio 65.4 % 67.6 % 7.4 % 51.7 % 73.4 % 54.3 % Acquisition expense ratio 16.6 % 14.7 % 9.5 % 14.3 % 21.5 % 15.2 % Other operating expense ratio 17.0 % 10.5 % 13.3 % 14.2 % 5.7 % 13.2 % Combined ratio 99.0 % 92.8 % 30.2 % 80.2 % 100.6 % 82.7 % Goodwill and intangible assets $ 20,724 $ — $ 564,634 $ 585,358 $ 7,650 $ 593,008 (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Three Months Ended June 30, 2017 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 743,902 $ 453,186 $ 336,226 $ 1,533,142 $ 152,813 $ 1,609,659 Premiums ceded (247,446 ) (115,262 ) (62,314 ) (424,850 ) (12,410 ) (360,964 ) Net premiums written 496,456 337,924 273,912 1,108,292 140,403 1,248,695 Change in unearned premiums 21,118 (23,222 ) (16,068 ) (18,172 ) 10,351 (7,821 ) Net premiums earned 517,574 314,702 257,844 1,090,120 150,754 1,240,874 Other underwriting income (loss) — (279 ) 4,277 3,998 824 4,822 Losses and loss adjustment expenses (350,939 ) (207,606 ) (20,694 ) (579,239 ) (110,621 ) (689,860 ) Acquisition expenses (78,872 ) (51,151 ) (25,666 ) (155,689 ) (34,747 ) (190,436 ) Other operating expenses (92,267 ) (36,711 ) (32,150 ) (161,128 ) (8,853 ) (169,981 ) Underwriting income (loss) $ (4,504 ) $ 18,955 $ 183,611 198,062 (2,643 ) 195,419 Net investment income 92,520 18,604 111,124 Net realized gains (losses) 18,046 3,689 21,735 Net impairment losses recognized in earnings (1,730 ) — (1,730 ) Equity in net income (loss) of investment funds accounted for using the equity method 32,706 — 32,706 Other income (loss) (1,994 ) — (1,994 ) Corporate expenses (2) (22,201 ) — (22,201 ) UGC transaction costs and other (2) (2,675 ) — (2,675 ) Amortization of intangible assets (30,824 ) — (30,824 ) Interest expense (25,912 ) (2,837 ) (28,749 ) Net foreign exchange gains (losses) (37,821 ) (1,722 ) (39,543 ) Income before income taxes 218,177 15,091 233,268 Income tax expense (34,169 ) — (34,169 ) Net income 184,008 15,091 199,099 Dividends attributable to redeemable noncontrolling interests — (4,586 ) (4,586 ) Amounts attributable to nonredeemable noncontrolling interests — (9,346 ) (9,346 ) Net income available to Arch 184,008 1,159 185,167 Preferred dividends (11,349 ) — (11,349 ) Net income available to Arch common shareholders $ 172,659 $ 1,159 $ 173,818 Underwriting Ratios Loss ratio 67.8 % 66.0 % 8.0 % 53.1 % 73.4 % 55.6 % Acquisition expense ratio 15.2 % 16.3 % 10.0 % 14.3 % 23.0 % 15.3 % Other operating expense ratio 17.8 % 11.7 % 12.5 % 14.8 % 5.9 % 13.7 % Combined ratio 100.8 % 94.0 % 30.5 % 82.2 % 102.3 % 84.6 % Goodwill and intangible assets $ 24,480 $ 609 $ 680,236 $ 705,325 $ 7,650 $ 712,975 (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Six Months Ended June 30, 2018 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 1,592,750 $ 1,067,810 $ 652,168 $ 3,312,807 $ 389,045 $ 3,534,758 Premiums ceded (492,445 ) (331,977 ) (97,004 ) (921,505 ) (68,907 ) (823,318 ) Net premiums written 1,100,305 735,833 555,164 2,391,302 320,138 2,711,440 Change in unearned premiums (15,119 ) (116,343 ) 15,556 (115,906 ) (23,872 ) (139,778 ) Net premiums earned 1,085,186 619,490 570,720 2,275,396 296,266 2,571,662 Other underwriting income (loss) — 1,103 6,731 7,834 1,389 9,223 Losses and loss adjustment expenses (711,195 ) (371,631 ) (65,057 ) (1,147,883 ) (215,130 ) (1,363,013 ) Acquisition expenses (175,839 ) (98,461 ) (54,304 ) (328,604 ) (65,610 ) (394,214 ) Other operating expenses (184,654 ) (71,249 ) (77,500 ) (333,403 ) (17,793 ) (351,196 ) Underwriting income (loss) $ 13,498 $ 79,252 $ 380,590 473,340 (878 ) 472,462 Net investment income 208,004 54,388 262,392 Net realized gains (losses) (171,404 ) (16,205 ) (187,609 ) Net impairment losses recognized in earnings (632 ) — (632 ) Equity in net income (loss) of investment funds accounted for using the equity method 36,541 — 36,541 Other income (loss) 3,187 — 3,187 Corporate expenses (2) (30,086 ) — (30,086 ) UGC transaction costs and other (2) (7,738 ) — (7,738 ) Amortization of intangible assets (53,208 ) — (53,208 ) Interest expense (51,965 ) (9,015 ) (60,980 ) Net foreign exchange gains (losses) 31,172 2,813 33,985 Income before income taxes 437,211 31,103 468,314 Income tax expense (45,556 ) (27 ) (45,583 ) Net income 391,655 31,076 422,731 Dividends attributable to redeemable noncontrolling interests — (9,170 ) (9,170 ) Amounts attributable to nonredeemable noncontrolling interests — (19,492 ) (19,492 ) Net income available to Arch 391,655 2,414 394,069 Preferred dividends (20,840 ) — (20,840 ) Loss on redemption of preferred shares (2,710 ) — (2,710 ) Net income available to Arch common shareholders $ 368,105 $ 2,414 $ 370,519 Underwriting Ratios Loss ratio 65.5 % 60.0 % 11.4 % 50.4 % 72.6 % 53.0 % Acquisition expense ratio 16.2 % 15.9 % 9.5 % 14.4 % 22.1 % 15.3 % Other operating expense ratio 17.0 % 11.5 % 13.6 % 14.7 % 6.0 % 13.7 % Combined ratio 98.7 % 87.4 % 34.5 % 79.5 % 100.7 % 82.0 % (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’ Six Months Ended June 30, 2017 Insurance Reinsurance Mortgage Sub-Total Other Total Gross premiums written (1) $ 1,526,183 $ 928,968 $ 684,849 $ 3,139,828 $ 306,933 $ 3,267,649 Premiums ceded (481,541 ) (281,354 ) (136,239 ) (898,962 ) (22,844 ) (742,694 ) Net premiums written 1,044,642 647,614 548,610 2,240,866 284,089 2,524,955 Change in unearned premiums (21,422 ) (88,061 ) (46,243 ) (155,726 ) (11,338 ) (167,064 ) Net premiums earned 1,023,220 559,553 502,367 2,085,140 272,751 2,357,891 Other underwriting income (loss) — (585 ) 8,400 7,815 1,640 9,455 Losses and loss adjustment expenses (683,580 ) (313,060 ) (49,759 ) (1,046,399 ) (196,031 ) (1,242,430 ) Acquisition expenses (153,740 ) (97,298 ) (54,432 ) (305,470 ) (67,255 ) (372,725 ) Other operating expenses (180,393 ) (74,244 ) (74,020 ) (328,657 ) (16,043 ) (344,700 ) Underwriting income (loss) $ 5,507 $ 74,366 $ 332,556 412,429 (4,938 ) 407,491 Net investment income 188,332 40,666 228,998 Net realized gains (losses) 46,558 9,330 55,888 Net impairment losses recognized in earnings (3,537 ) — (3,537 ) Equity in net income (loss) of investment funds accounted for using the equity method 80,794 — 80,794 Other income (loss) (2,776 ) — (2,776 ) Corporate expenses (2) (34,409 ) — (34,409 ) UGC transaction costs and other (2) (18,259 ) — (18,259 ) Amortization of intangible assets (62,118 ) — (62,118 ) Interest expense (51,668 ) (5,757 ) (57,425 ) Net foreign exchange gains (losses) (57,666 ) (1,281 ) (58,947 ) Income before income taxes 497,680 38,020 535,700 Income tax expense (62,566 ) — (62,566 ) Net income 435,114 38,020 473,134 Dividends attributable to redeemable noncontrolling interests — (9,170 ) (9,170 ) Amounts attributable to nonredeemable noncontrolling interests — (25,670 ) (25,670 ) Net income available to Arch 435,114 3,180 438,294 Preferred dividends (22,567 ) — (22,567 ) Net income available to Arch common shareholders $ 412,547 $ 3,180 $ 415,727 Underwriting Ratios Loss ratio 66.8 % 55.9 % 9.9 % 50.2 % 71.9 % 52.7 % Acquisition expense ratio 15.0 % 17.4 % 10.8 % 14.6 % 24.7 % 15.8 % Other operating expense ratio 17.6 % 13.3 % 14.7 % 15.8 % 5.9 % 14.6 % Combined ratio 99.4 % 86.6 % 35.4 % 80.6 % 102.5 % 83.1 % (1) Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total. (2) Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’

Reserve for Losses and Loss A27

Reserve for Losses and Loss Adjustment Expenses (Tables)	6 Months Ended
	Jun. 30, 2018
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Analysis of losses and loss adjustment expenses and reconciliation of beginning and ending reserve balances	The following table represents an analysis of losses and loss adjustment expenses and a reconciliation of the beginning and ending reserve for losses and loss adjustment expenses: Three Months Ended Six Months Ended June 30, June 30, 2018 2017 2018 2017 Reserve for losses and loss adjustment expenses at beginning of period $ 11,496,205 $ 10,296,821 $ 11,383,792 $ 10,200,960 Unpaid losses and loss adjustment expenses recoverable 2,446,990 2,095,130 2,464,910 2,083,575 Net reserve for losses and loss adjustment expenses at beginning of period 9,049,215 8,201,691 8,918,882 8,117,385 Net incurred losses and loss adjustment expenses relating to losses occurring in: Current year 790,742 759,261 1,478,627 1,395,037 Prior years (64,589 ) (69,401 ) (115,614 ) (152,607 ) Total net incurred losses and loss adjustment expenses 726,153 689,860 1,363,013 1,242,430 Retroactive reinsurance transaction (1) (420,404 ) — (420,404 ) — Net foreign exchange (gains) losses (118,542 ) 75,295 (74,528 ) 106,574 Net paid losses and loss adjustment expenses relating to losses occurring in: Current year (59,022 ) (80,499 ) (95,022 ) (115,502 ) Prior years (404,812 ) (482,046 ) (919,353 ) (946,586 ) Total net paid losses and loss adjustment expenses (463,834 ) (562,545 ) (1,014,375 ) (1,062,088 ) Net reserve for losses and loss adjustment expenses at end of period 8,772,588 8,404,301 8,772,588 8,404,301 Unpaid losses and loss adjustment expenses recoverable 2,651,749 2,116,210 2,651,749 2,116,210 Reserve for losses and loss adjustment expenses at end of period $ 11,424,337 $ 10,520,511 $ 11,424,337 $ 10,520,511 (1) During the 2018 second quarter, a subsidiary of the Company entered into a retroactive reinsurance transaction with a third party reinsurer to reinsure run-off liabilities associated with certain discontinued U.S. specialty casualty and program exposures.

Investment Information (Tables)

Investment Information (Tables)	6 Months Ended
	Jun. 30, 2018
Disclosure Investment Information [Abstract]
Summary of fair value and cost or amortized cost of available for sale securities	The following table summarizes the fair value and cost or amortized cost of the Company’s securities classified as available for sale: Estimated Fair Value Gross Unrealized Gains Gross Unrealized Losses Cost or Amortized Cost OTTI Unrealized Losses (2) June 30, 2018 Fixed maturities (1): Corporate bonds $ 5,535,763 $ 7,199 $ (103,978 ) $ 5,632,542 $ (69 ) Mortgage backed securities 488,699 1,699 (4,742 ) 491,742 (15 ) Municipal bonds 1,431,256 7,371 (22,047 ) 1,445,932 — Commercial mortgage backed securities 590,198 776 (13,222 ) 602,644 — U.S. government and government agencies 2,788,272 6,216 (19,577 ) 2,801,633 — Non-U.S. government securities 1,692,783 17,124 (27,480 ) 1,703,139 — Asset backed securities 1,820,489 3,310 (16,574 ) 1,833,753 — Total 14,347,460 43,695 (207,620 ) 14,511,385 (84 ) Equity securities (3) Other investments — — — — — Short-term investments 1,096,798 338 (72 ) 1,096,532 — Total $ 15,444,258 $ 44,033 $ (207,692 ) $ 15,607,917 $ (84 ) December 31, 2017 Fixed maturities (1): Corporate bonds $ 4,434,439 $ 30,943 $ (32,340 ) $ 4,435,836 $ (73 ) Mortgage backed securities 316,141 1,640 (2,561 ) 317,062 (15 ) Municipal bonds 2,158,840 20,285 (12,308 ) 2,150,863 — Commercial mortgage backed securities 545,817 2,131 (4,268 ) 547,954 — U.S. government and government agencies 3,484,257 2,188 (28,769 ) 3,510,838 — Non-U.S. government securities 1,612,754 48,764 (17,321 ) 1,581,311 — Asset backed securities 1,780,143 5,147 (8,614 ) 1,783,610 — Total 14,332,391 111,098 (106,181 ) 14,327,474 (88 ) Equity securities 504,333 88,739 (5,583 ) 421,177 — Other investments 264,989 66,946 (120 ) 198,163 — Short-term investments 1,469,042 650 (563 ) 1,468,955 — Total $ 16,570,755 $ 267,433 $ (112,447 ) $ 16,415,769 $ (88 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” (2) Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018 , the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million , compared to a net unrealized gain of $0.3 million at December 31, 2017 . (3) Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2 ). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table.
Summary of available for sale securities in a continual unrealized loss position	The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position: Less than 12 Months 12 Months or More Total Estimated Gross Unrealized Losses Estimated Gross Unrealized Losses Estimated Gross Unrealized Losses June 30, 2018 Fixed maturities (1): Corporate bonds $ 4,541,077 $ (94,563 ) $ 236,332 $ (9,415 ) $ 4,777,409 $ (103,978 ) Mortgage backed securities 296,876 (4,618 ) 708 (124 ) 297,584 (4,742 ) Municipal bonds 814,694 (16,888 ) 109,913 (5,159 ) 924,607 (22,047 ) Commercial mortgage backed securities 351,013 (9,067 ) 56,382 (4,155 ) 407,395 (13,222 ) U.S. government and government agencies 1,957,338 (18,470 ) 41,069 (1,107 ) 1,998,407 (19,577 ) Non-U.S. government securities 1,243,055 (24,034 ) 160,434 (3,446 ) 1,403,489 (27,480 ) Asset backed securities 1,377,269 (14,096 ) 110,507 (2,478 ) 1,487,776 (16,574 ) Total 10,581,322 (181,736 ) 715,345 (25,884 ) 11,296,667 (207,620 ) Equity securities (2) Other investments — — — — — — Short-term investments 53,937 (72 ) — — 53,937 (72 ) Total $ 10,635,259 $ (181,808 ) $ 715,345 $ (25,884 ) $ 11,350,604 $ (207,692 ) December 31, 2017 Fixed maturities (1): Corporate bonds $ 2,320,716 $ (25,411 ) $ 279,082 $ (6,929 ) $ 2,599,798 $ (32,340 ) Mortgage backed securities 221,113 (1,715 ) 28,380 (846 ) 249,493 (2,561 ) Municipal bonds 1,030,389 (8,438 ) 132,469 (3,870 ) 1,162,858 (12,308 ) Commercial mortgage backed securities 225,164 (1,899 ) 57,291 (2,369 ) 282,455 (4,268 ) U.S. government and government agencies 2,646,415 (26,501 ) 111,879 (2,268 ) 2,758,294 (28,769 ) Non-U.S. government securities 1,218,514 (15,546 ) 93,530 (1,775 ) 1,312,044 (17,321 ) Asset backed securities 1,111,246 (5,915 ) 209,207 (2,699 ) 1,320,453 (8,614 ) Total 8,773,557 (85,425 ) 911,838 (20,756 ) 9,685,395 (106,181 ) Equity securities 166,562 (5,583 ) — — 166,562 (5,583 ) Other investments 15,025 (120 ) — — 15,025 (120 ) Short-term investments 109,528 (563 ) — — 109,528 (563 ) Total $ 9,064,672 $ (91,691 ) $ 911,838 $ (20,756 ) $ 9,976,510 $ (112,447 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” (2) Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2 ). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table.
Contractual maturities of the Company's fixed maturities and fixed maturities pledged under securities lending arrangements	The contractual maturities of the Company’s fixed maturities are shown in the following table. Expected maturities, which are management’s best estimates, will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. June 30, 2018 December 31, 2017 Maturity Estimated Fair Value Amortized Cost Estimated Amortized Cost Due in one year or less $ 344,007 $ 343,938 $ 550,711 $ 548,771 Due after one year through five years 8,154,773 8,235,481 7,436,153 7,434,801 Due after five years through 10 years 2,703,341 2,756,097 3,369,635 3,369,750 Due after 10 years 245,953 247,730 333,791 325,526 11,448,074 11,583,246 11,690,290 11,678,848 Mortgage backed securities 488,699 491,742 316,141 317,062 Commercial mortgage backed securities 590,198 602,644 545,817 547,954 Asset backed securities 1,820,489 1,833,753 1,780,143 1,783,610 Total (1) $ 14,347,460 $ 14,511,385 $ 14,332,391 $ 14,327,474 (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”
Securities lending transactions accounted for as secured borrowings, by significant investment category	The Company’s securities lending transactions were accounted for as secured borrowings with significant investment categories as follows: Remaining Contractual Maturity of the Agreements Overnight and Continuous Less than 30 Days 30-90 Days 90 Days or More Total June 30, 2018 U.S. government and government agencies $ 147,509 $ — $ 48,268 $ — $ 195,777 Corporate bonds 29,265 — — — 29,265 Equity securities 11,906 — — — 11,906 Total $ 188,680 $ — $ 48,268 $ — $ 236,948 Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 9 $ — Amounts related to securities lending not included in offsetting disclosure in Note 9 $ 236,948 December 31, 2017 U.S. government and government agencies $ 343,425 $ 20,309 $ 76,086 $ — $ 439,820 Corporate bonds 28,003 — — — 28,003 Equity securities 8,782 — — — 8,782 Total $ 380,210 $ 20,309 $ 76,086 $ — $ 476,605 Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 9 $ — Amounts related to securities lending not included in offsetting disclosure in Note 9 $ 476,605
Summary of other investments, including available for sale and fair value option components	The following table summarizes the Company’s other investments, including available for sale and fair value option components: June 30, December 31, Available for sale securities: Asian and emerging markets $ — $ 135,140 Investment grade fixed income — 53,878 Credit related funds — 18,365 Other — 57,606 Total available for sale (1) — 264,989 Fair value option: Term loan investments (par value: $1,321,154 and $1,223,453) $ 1,286,305 $ 1,200,882 Mezzanine debt funds 257,750 252,160 Credit related funds 199,437 175,422 Investment grade fixed income 98,240 102,347 Asian and emerging markets 350,308 258,541 Other (2) 136,864 147,029 Total fair value option 2,328,904 2,136,381 Total $ 2,328,904 $ 2,401,370 (1) The Company reviewed the accounting treatment for three limited partnership investments which were accounted for as available for sale at December 31, 2017 during the 2018 first quarter and determined, based on reconsideration during the period of the Company’s percentage ownership, that the equity method of accounting was appropriate for such investments. (2) Includes fund investments with strategies in mortgage servicing rights, transportation, infrastructure assets and other.
Summary of assets and liabilities accounted for using the fair value option	The following table summarizes the Company’s assets and liabilities which are accounted for using the fair value option: June 30, December 31, Fixed maturities $ 1,428,503 $ 1,642,855 Other investments 2,328,904 2,136,381 Short-term investments 217,066 297,426 Equity securities 137,138 139,575 Investments accounted for using the fair value option $ 4,111,611 $ 4,216,237
Summary of investments in limited partnership interests where the Company has a variable interest	The following table summarizes investments in limited partnership interests where the Company has a variable interest by balance sheet line item: June 30, December 31, Investments accounted for using the equity method (1) $ 1,292,573 $ 1,041,321 Investments accounted for using the fair value option (2) 130,471 130,471 Total $ 1,423,044 $ 1,171,792 (1) Aggregate unfunded commitments were $994.7 million at June 30, 2018 , compared to $1.02 billion at December 31, 2017 . (2) Aggregate unfunded commitments were $91.0 million at June 30, 2018 , compared to $100.4 million at December 31, 2017 .
Components of net investment income	The components of net investment income were derived from the following sources: June 30, 2018 2017 Three Months Ended Fixed maturities $ 115,110 $ 94,270 Equity securities 4,777 3,654 Short-term investments 4,392 2,016 Other (1) 38,168 34,076 Gross investment income 162,447 134,016 Investment expenses (26,779 ) (22,892 ) Net investment income $ 135,668 $ 111,124 Six Months Ended Fixed maturities $ 222,997 $ 188,663 Equity securities 7,345 6,297 Short-term investments 9,252 3,775 Other (1) 75,542 73,656 Gross investment income 315,136 272,391 Investment expenses (52,744 ) (43,393 ) Net investment income $ 262,392 $ 228,998 (1) Includes income distributions from investment funds, term loan investments and other items.
Summary of net realized gains (losses), excluding other-than-temporary impairment provisions	Net realized gains (losses) were as follows, excluding net impairment losses recognized in earnings: June 30, 2018 2017 Three Months Ended Available for sale securities: Gross gains on investment sales $ 18,777 $ 76,730 Gross losses on investment sales (57,711 ) (52,619 ) Change in fair value of assets and liabilities accounted for using the fair value option: Fixed maturities (22,927 ) 9,656 Other investments (254 ) 637 Equity securities 1,230 2,829 Short-term investments (136 ) 3,328 Equity securities, at fair value (1): Net realized gains (losses) on sales during the period (5,918 ) — Net unrealized gains (losses) on equity securities still held at reporting date (7,278 ) — Derivative instruments (2) (2,146 ) (4,770 ) Other (3) (248 ) (14,056 ) Net realized gains (losses) $ (76,611 ) $ 21,735 Six Months Ended Available for sale securities: Gross gains on investment sales $ 33,742 $ 145,905 Gross losses on investment sales (140,262 ) (113,981 ) Change in fair value of assets and liabilities accounted for using the fair value option: Fixed maturities (40,478 ) 30,197 Other investments (6,628 ) 17,885 Equity securities 7,898 6,374 Short-term investments (287 ) 3,332 Equity securities, at fair value (1): Net realized gains (losses) on sales during the period (11,286 ) — Net unrealized gains (losses) on equity securities still held at reporting date (14,861 ) — Derivative instruments (2) (6,109 ) (13,951 ) Other (3) (9,338 ) (19,873 ) Net realized gains (losses) $ (187,609 ) $ 55,888 (1) Pursuant to new accounting guidance (see Note 2), changes in fair value on equity securities are recorded through net income effective January 1, 2018. (2) See Note 9 for information on the Company’s derivative instruments. (3) Includes the re-measurement of contingent consideration liability amounts.
Summary of OTTI recognized in earnings by asset class	The following table details the net impairment losses recognized in earnings by asset class: June 30, 2018 2017 Three Months Ended Fixed maturities: Mortgage backed securities $ (81 ) $ (92 ) Corporate bonds (241 ) (1,401 ) Asset backed securities (148 ) — Municipal bonds — (173 ) Total (470 ) (1,666 ) Other investments — (64 ) Net impairment losses recognized in earnings $ (470 ) $ (1,730 ) Six Months Ended Fixed maturities: Mortgage backed securities $ (123 ) $ (1,411 ) Corporate bonds (361 ) (1,402 ) Non-U.S. government securities — (198 ) Asset backed securities (148 ) — Municipal bonds — (173 ) Total (632 ) (3,184 ) Equity securities — (186 ) Other investments — (167 ) Net impairment losses recognized in earnings $ (632 ) $ (3,537 )
Rollforward of the amount related to credit losses recognized in earnings for which a portion was recognized in AOCI	The following table provides a roll forward of the amount related to credit losses recognized in earnings for which a portion of an OTTI was recognized in accumulated other comprehensive income: June 30, 2018 2017 Three Months Ended Balance at start of period $ 767 $ 12,537 Credit loss impairments recognized on securities not previously impaired — 31 Credit loss impairments recognized on securities previously impaired — 172 Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security — — Reductions for securities sold during the period (69 ) (8,303 ) Balance at end of period $ 698 $ 4,437 Six Months Ended Balance at start of year $ 767 $ 13,138 Credit loss impairments recognized on securities not previously impaired — 31 Credit loss impairments recognized on securities previously impaired — 195 Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security — — Reductions for securities sold during the period (69 ) (8,927 ) Balance at end of period $ 698 $ 4,437
Summary of restricted assets	The following table details the value of the Company’s restricted assets: June 30, December 31, Assets used for collateral or guarantees: Affiliated transactions $ 4,549,122 $ 4,323,726 Third party agreements 1,537,336 1,674,304 Deposits with U.S. regulatory authorities 701,026 616,987 Deposits with non-U.S. regulatory authorities 58,371 55,895 Total restricted assets $ 6,845,855 $ 6,670,912

Fair Value (Tables)

Fair Value (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair value hierarchy	The following table presents the Company’s financial assets and liabilities measured at fair value by level at June 30, 2018 : Estimated Fair Value Measurements Using: Estimated Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets measured at fair value (1): Available for sale securities: Fixed maturities: Corporate bonds $ 5,535,763 $ — $ 5,526,990 $ 8,773 Mortgage backed securities 488,699 — 488,352 347 Municipal bonds 1,431,256 — 1,431,256 — Commercial mortgage backed securities 590,198 — 590,169 29 U.S. government and government agencies 2,788,272 2,660,621 127,651 — Non-U.S. government securities 1,692,783 — 1,692,783 — Asset backed securities 1,820,489 — 1,820,489 — Total 14,347,460 2,660,621 11,677,690 9,149 Short-term investments 1,096,798 1,053,661 43,137 — Equity securities, at fair value 546,075 493,975 52,100 — Derivative instruments (4) 31,623 — 31,623 — Fair value option: Corporate bonds 936,840 — 925,505 11,335 Non-U.S. government bonds 132,162 — 132,162 — Mortgage backed securities 16,410 — 16,410 — Municipal bonds 7,082 — 7,082 — Commercial mortgage backed securities 1,306 — 1,306 — Asset backed securities 214,718 — 214,718 — U.S. government and government agencies 119,985 119,875 110 — Short-term investments 217,066 19,869 197,197 — Equity securities 137,138 65,400 71,738 — Other investments 1,260,645 65,665 1,136,766 58,214 Other investments measured at net asset value (2) 1,068,259 Total 4,111,611 270,809 2,702,994 69,549 Total assets measured at fair value $ 20,133,567 $ 4,479,066 $ 14,507,544 $ 78,698 Liabilities measured at fair value: Contingent consideration liabilities $ (63,930 ) $ — $ — $ (63,930 ) Securities sold but not yet purchased (3) (24,529 ) — (24,529 ) — Derivative instruments (4) (22,398 ) — (22,398 ) — Total liabilities measured at fair value $ (110,857 ) $ — $ (46,927 ) $ (63,930 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See Note 7 , “Investment Information—Securities Lending Agreements.” (2) In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (3) Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets. (4) See Note 9 , “Derivative Instruments.” The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2017 : Estimated Fair Value Measurements Using: Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets measured at fair value (1): Available for sale securities: Fixed maturities: Corporate bonds $ 4,434,439 $ — $ 4,424,979 $ 9,460 Mortgage backed securities 316,141 — 315,754 387 Municipal bonds 2,158,840 — 2,158,840 — Commercial mortgage backed securities 545,817 — 545,277 540 U.S. government and government agencies 3,484,257 3,408,902 75,355 — Non-U.S. government securities 1,612,754 — 1,612,754 — Asset backed securities 1,780,143 — 1,775,143 5,000 Total 14,332,391 3,408,902 10,908,102 15,387 Equity securities 504,333 498,182 6,151 — Short-term investments 1,469,042 1,420,732 48,310 — Other investments 76,427 74,611 1,816 — Other investments measured at net asset value (2) 188,562 Total other investments 264,989 74,611 1,816 — Derivative instruments (4) 15,747 — 15,747 — Fair value option: Corporate bonds 1,068,725 — 1,056,508 12,217 Non-U.S. government bonds 195,788 — 195,788 — Mortgage backed securities 20,491 — 20,491 — Municipal bonds 15,210 — 15,210 — Commercial mortgage backed securities 11,997 — 11,997 — Asset backed securities 99,354 — 99,354 — U.S. government and government agencies 231,290 231,019 271 — Short-term investments 297,426 40,166 257,260 — Equity securities 139,575 67,440 72,135 — Other investments 1,128,094 82,291 986,636 59,167 Other investments measured at net asset value (2) 1,008,287 Total 4,216,237 420,916 2,715,650 71,384 Total assets measured at fair value $ 20,802,739 $ 5,823,343 $ 13,695,776 $ 86,771 Liabilities measured at fair value: Contingent consideration liabilities $ (60,996 ) $ — $ — $ (60,996 ) Securities sold but not yet purchased (3) (34,375 ) — (34,375 ) — Derivative instruments (4) (20,464 ) — (20,464 ) — Total liabilities measured at fair value $ (115,835 ) $ — $ (54,839 ) $ (60,996 ) (1) In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See Note 7 , “Investment Information—Securities Lending Agreements.” (2) In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (3) Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets. (4) See Note 9 , “Derivative Instruments.”
Rollforward of Level 3 investments	The following table presents a reconciliation of the beginning and ending balances for all financial assets and liabilities measured at fair value on a recurring basis using Level 3 inputs: Assets Liabilities s Available For Sale Fair Value Option Structured Securities (1) Corporate Corporate Other Investments Total Contingent Consideration Liabilities Three Months Ended June 30, 2018 Balance at beginning of period $ 5,413 $ 9,152 $ 11,872 $ 58,452 $ 84,889 $ (62,449 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3 — (537 ) 336 (198 ) (1,481 ) Included in other comprehensive income (4 ) (316 ) — — (320 ) — Purchases, issuances, sales and settlements Purchases — 393 — — 393 — Issuances — — — — — — Sales (5,003 ) — — (74 ) (5,077 ) — Settlements (33 ) (456 ) — (500 ) (989 ) — Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ 376 $ 8,773 $ 11,335 $ 58,214 $ 78,698 $ (63,930 ) Three Months Ended June 30, 2017 Balance at beginning of period $ 10,637 $ 18,601 $ — $ 25,000 $ 54,238 $ (125,544 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3,072 636 — — 3,708 (3,441 ) Included in other comprehensive income — — — — — — Purchases, issuances, sales and settlements Purchases — 4,935 — — 4,935 — Issuances — — — — — — Sales (13,640 ) (12,602 ) — — (26,242 ) — Settlements (69 ) — — — (69 ) 71,739 Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ — $ 11,570 $ — $ 25,000 $ 36,570 $ (57,246 ) Six Months Ended June 30, 2018 Balance at beginning of year $ 5,927 $ 9,460 $ 12,217 $ 59,167 $ 86,771 $ (60,996 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 4 — (612 ) (379 ) (987 ) (2,934 ) Included in other comprehensive income (8 ) (168 ) — — (176 ) — Purchases, issuances, sales and settlements Purchases — 393 — — 393 — Issuances — — — — — — Sales (5,003 ) — — (74 ) (5,077 ) — Settlements (544 ) (912 ) (270 ) (500 ) (2,226 ) — Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ 376 $ 8,773 $ 11,335 $ 58,214 $ 78,698 $ (63,930 ) Six Months Ended June 30, 2017 Balance at beginning of year $ 11,289 $ 18,344 $ — $ 25,000 $ 54,633 $ (122,350 ) Total gains or (losses) (realized/unrealized) Included in earnings (2) 3,779 893 — — 4,672 (7,087 ) Included in other comprehensive income — — — — — — Purchases, issuances, sales and settlements Purchases — 4,935 — — 4,935 — Issuances — — — — — — Sales (13,640 ) (12,602 ) — — (26,242 ) — Settlements (1,428 ) — — — (1,428 ) 72,191 Transfers in and/or out of Level 3 — — — — — — Balance at end of period $ — $ 11,570 $ — $ 25,000 $ 36,570 $ (57,246 ) (1) Includes asset backed securities, mortgage backed securities and commercial mortgage backed securities. (2) Gains or losses were included in net realized gains (losses).

Derivative Instruments (Tables)

Derivative Instruments (Tables)	6 Months Ended
	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair value and notional amount of derivatives	The following table summarizes information on the fair values and notional values of the Company’s derivative instruments: Estimated Fair Value Asset Derivatives Liability Derivatives Notional Value (1) June 30, 2018 Futures contracts (2) $ 556 $ (3,425 ) $ 1,276,189 Foreign currency forward contracts (2) 18,183 (7,732 ) 1,072,743 TBAs (3) 14,761 — 14,480 Other (2) 12,884 (11,241 ) 1,924,299 Total $ 46,384 $ (22,398 ) December 31, 2017 Futures contracts (2) $ 3,371 $ (1,542 ) $ 1,452,497 Foreign currency forward contracts (2) 4,478 (4,381 ) 686,941 TBAs (3) 27,184 — 27,066 Other (2) 7,898 (14,541 ) 1,457,345 Total $ 42,931 $ (20,464 ) (1) Represents the absolute notional value of all outstanding contracts, consisting of long and short positions. (2) The fair value of asset derivatives are included in ‘other assets’ and the fair value of liability derivatives are included in ‘other liabilities.’ (3) The fair value of TBAs are included in ‘fixed maturities available for sale, at fair value.’
Summary of net realized gains (losses) recorded in the consolidated statements of income	Realized and unrealized contract gains and losses on the Company’s derivative instruments are reflected in ‘net realized gains (losses)’ in the consolidated statements of income, as summarized in the following table: Derivatives not designated as June 30, hedging instruments: 2018 2017 Three Months Ended Net realized gains (losses): Futures contracts $ (240 ) $ (5,310 ) Foreign currency forward contracts (1,692 ) (272 ) TBAs — 86 Other (214 ) 726 Total $ (2,146 ) $ (4,770 ) Six Months Ended Net realized gains (losses): Futures contracts $ 4,790 $ 2,410 Foreign currency forward contracts (7,616 ) (12,038 ) TBAs (97 ) 21 Other (3,186 ) (4,344 ) Total $ (6,109 ) $ (13,951 )

Variable Interest Entities an31

Variable Interest Entities and Noncontrolling Interests (Tables)	6 Months Ended
	Jun. 30, 2018
Noncontrolling Interest [Abstract]
Carrying value of assets and liabilities of variable interest entity	The following table provides the carrying amount and balance sheet caption in which the assets and liabilities of Watford Re are reported: June 30, December 31, 2018 2017 Assets Investments accounted for using the fair value option $ 2,385,316 $ 2,426,066 Fixed maturities available for sale, at fair value 279,177 — Equity securities, at fair value 63,010 — Cash 45,644 54,503 Accrued investment income 18,108 18,261 Premiums receivable 232,842 177,492 Reinsurance recoverable on unpaid and paid losses and LAE 54,299 42,777 Ceded unearned premiums 55,853 24,762 Deferred acquisition costs 88,752 85,961 Receivable for securities sold 31,387 36,374 Goodwill and intangible assets 7,650 7,650 Other assets 75,478 140,808 Total assets of consolidated VIE $ 3,337,516 $ 3,014,654 Liabilities Reserves for losses and loss adjustment expenses $ 899,395 $ 798,262 Unearned premiums 384,537 330,644 Reinsurance balances payable 41,185 18,424 Revolving credit agreement borrowings 447,289 441,132 Payable for securities purchased 132,164 42,501 Other liabilities 244,320 215,186 Total liabilities of consolidated VIE $ 2,148,890 $ 1,846,149 Redeemable noncontrolling interests $ 220,805 $ 220,622
Activity in non-redeemable noncontrolling interests	The following table sets forth activity in the non-redeemable noncontrolling interests: June 30, 2018 2017 Three Months Ended Balance, beginning of period $ 854,112 $ 868,186 Amounts attributable to noncontrolling interests 8,116 9,346 Other comprehensive income attributable to noncontrolling interests (1,075 ) (76 ) Balance, end of period $ 861,153 $ 877,456 Six Months Ended Balance, beginning of year $ 843,411 $ 851,854 Amounts attributable to noncontrolling interests 19,492 25,670 Other comprehensive income attributable to noncontrolling interests (1,750 ) (68 ) Balance, end of period $ 861,153 $ 877,456
Activity in redeemable noncontrolling interests	The following table sets forth activity in the redeemable non-controlling interests: June 30, 2018 2017 Three Months Ended Balance, beginning of period $ 206,013 $ 205,644 Accretion of preference share issuance costs 92 92 Balance, end of period $ 206,105 $ 205,736 Six Months Ended Balance, beginning of year $ 205,922 $ 205,553 Accretion of preference share issuance costs 183 183 Balance, end of period $ 206,105 $ 205,736
Portion of income or loss attributable to third party investors	The portion of Watford Re’s income or loss attributable to third party investors, recorded in the Company’s consolidated statements of income in ‘net (income) loss attributable to noncontrolling interests,’ are summarized in the table below: June 30, 2018 2017 Three Months Ended Amounts attributable to non-redeemable noncontrolling interests $ (8,116 ) $ (9,346 ) Dividends attributable to redeemable noncontrolling interests (4,585 ) (4,586 ) Net (income) loss attributable to noncontrolling interests $ (12,701 ) $ (13,932 ) Six Months Ended Amounts attributable to non-redeemable noncontrolling interests $ (19,492 ) $ (25,670 ) Dividends attributable to redeemable noncontrolling interests (9,170 ) (9,170 ) Net (income) loss attributable to noncontrolling interests $ (28,662 ) $ (34,840 )
Total assets and maximum exposure to loss associated with VIEs	Maximum Exposure to Loss Total VIE Assets On-Balance Sheet Off-Balance Sheet Total June 30, 2018 Bellemeade Re I Ltd. $ 69,079 $ 263 $ 469 $ 732 Bellemeade Re II Ltd. 55,388 14 110 124 Bellemeade 2017-1 Ltd. 329,030 815 1,759 2,574 Bellemeade 2018-1 Ltd. 374,460 1,242 2,050 3,292 Total $ 827,957 $ 2,334 $ 4,388 $ 6,722 December 31, 2017 Bellemeade Re I Ltd. $ 92,390 $ 471 $ 832 $ 1,303 Bellemeade Re II Ltd. 135,201 20 527 547 Bellemeade 2017-1 Ltd. 347,139 391 1,867 2,258 Total $ 574,730 $ 882 $ 3,226 $ 4,108

Other Comprehensive Income (L32

Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2018
Comprehensive Income Note Disclosure [Abstract]
Details about amounts reclassified from AOCI	The following tables present details about amounts reclassified from accumulated other comprehensive income and the tax effects allocated to each component of other comprehensive income (loss): Amounts Reclassified from AOCI Consolidated Statement of Income Three Months Ended Six Months Ended Details About Line Item That Includes June 30, June 30, AOCI Components Reclassification 2018 2017 2018 2017 Unrealized appreciation on available-for-sale investments Net realized gains (losses) $ (38,935 ) $ 24,111 $ (106,521 ) $ 31,924 Other-than-temporary impairment losses (470 ) (1,730 ) (632 ) (3,537 ) Total before tax (39,405 ) 22,381 (107,153 ) 28,387 Income tax (expense) benefit 2,762 (5,157 ) 8,049 (6,119 ) Net of tax $ (36,643 ) $ 17,224 $ (99,104 ) $ 22,268
Schedule of comprehensive income (loss)	Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Three Months Ended June 30, 2018 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ (88,034 ) $ (2,763 ) $ (85,271 ) Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income (39,405 ) (2,762 ) (36,643 ) Foreign currency translation adjustments (12,701 ) (106 ) (12,595 ) Other comprehensive income (loss) $ (61,330 ) $ (107 ) $ (61,223 ) Three Months Ended June 30, 2017 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ 108,011 $ 15,042 $ 92,969 Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income 22,381 5,157 17,224 Foreign currency translation adjustments 18,509 212 18,297 Other comprehensive income (loss) $ 104,139 $ 10,097 $ 94,042 Six Months Ended June 30, 2018 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ (277,977 ) $ (26,029 ) $ (251,948 ) Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income (107,153 ) (8,049 ) (99,104 ) Foreign currency translation adjustments (11,269 ) 44 (11,313 ) Other comprehensive income (loss) $ (182,093 ) $ (17,936 ) $ (164,157 ) Six Months Ended June 30, 2017 Unrealized appreciation (decline) in value of investments: Unrealized holding gains (losses) arising during period $ 219,483 $ 25,722 $ 193,761 Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss) — — — Less reclassification of net realized gains (losses) included in net income 28,387 6,119 22,268 Foreign currency translation adjustments 21,674 253 21,421 Other comprehensive income (loss) $ 212,770 $ 19,856 $ 192,914

Guarantor Financial Informati33

Guarantor Financial Information (Tables)	6 Months Ended
	Jun. 30, 2018
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed consolidating balance sheet	June 30, 2018 Condensed Consolidating Balance Sheet Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Assets Total investments $ 134 $ 198,122 $ 21,353,862 $ (14,700 ) $ 21,537,418 Cash 7,033 27,513 492,082 — 526,628 Investments in subsidiaries 9,455,746 4,033,568 — (13,489,314 ) — Due from subsidiaries and affiliates — 2 1,849,468 (1,849,470 ) — Premiums receivable — — 1,897,517 (546,207 ) 1,351,310 Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses — — 8,441,528 (5,714,225 ) 2,727,303 Contractholder receivables — — 2,044,322 — 2,044,322 Ceded unearned premiums — — 1,821,539 (806,876 ) 1,014,663 Deferred acquisition costs — — 633,454 (63,637 ) 569,817 Goodwill and intangible assets — — 593,008 — 593,008 Other assets 12,978 56,042 1,555,017 (135,386 ) 1,488,651 Total assets $ 9,475,891 $ 4,315,247 $ 40,681,797 $ (22,619,815 ) $ 31,853,120 Liabilities Reserve for losses and loss adjustment expenses $ — $ — $ 16,855,050 $ (5,430,713 ) $ 11,424,337 Unearned premiums — — 4,640,416 (806,876 ) 3,833,540 Reinsurance balances payable — — 957,289 (546,207 ) 411,082 Contractholder payables — — 2,044,322 — 2,044,322 Collateral held for insured obligations — — 257,396 257,396 Senior notes 297,101 494,672 941,438 — 1,733,211 Revolving credit agreement borrowings — — 572,289 — 572,289 Due to subsidiaries and affiliates 1,824 536,747 1,310,899 (1,849,470 ) — Other liabilities 13,211 29,304 1,785,950 (482,535 ) 1,345,930 Total liabilities 312,136 1,060,723 29,365,049 (9,115,801 ) 21,622,107 Redeemable noncontrolling interests — — 220,805 (14,700 ) 206,105 Shareholders’ Equity Total shareholders’ equity available to Arch 9,163,755 3,254,524 10,234,790 (13,489,314 ) 9,163,755 Non-redeemable noncontrolling interests — — 861,153 — 861,153 Total shareholders’ equity 9,163,755 3,254,524 11,095,943 (13,489,314 ) 10,024,908 Total liabilities, noncontrolling interests and shareholders’ equity $ 9,475,891 $ 4,315,247 $ 40,681,797 $ (22,619,815 ) $ 31,853,120 December 31, 2017 Condensed Consolidating Balance Sheet Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Assets Total investments $ 96,540 $ 46,281 $ 21,711,891 $ (14,700 ) $ 21,840,012 Cash 9,997 30,380 565,822 — 606,199 Investments in subsidiaries 9,396,621 4,097,765 — (13,494,386 ) — Due from subsidiaries and affiliates 394 — 1,828,864 (1,829,258 ) — Premiums receivable — — 2,967,701 (1,832,452 ) 1,135,249 Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses — — 8,442,192 (5,902,049 ) 2,540,143 Contractholder receivables — — 1,978,414 — 1,978,414 Ceded unearned premiums — — 2,165,789 (1,239,178 ) 926,611 Deferred acquisition costs — — 693,053 (157,229 ) 535,824 Goodwill and intangible assets — — 652,611 — 652,611 Other assets 13,176 49,585 1,860,505 (86,671 ) 1,836,595 Total assets $ 9,516,728 $ 4,224,011 $ 42,866,842 $ (24,555,923 ) $ 32,051,658 Liabilities Reserve for losses and loss adjustment expenses $ — $ — $ 17,236,401 $ (5,852,609 ) $ 11,383,792 Unearned premiums — — 4,861,491 (1,239,177 ) 3,622,314 Reinsurance balances payable — — 2,155,947 (1,832,451 ) 323,496 Contractholder payables — — 1,978,414 — 1,978,414 Collateral held for insured obligations — — 240,183 — 240,183 Senior notes 297,053 494,621 941,210 — 1,732,884 Revolving credit agreement borrowings — — 816,132 — 816,132 Due to subsidiaries and affiliates 235 536,919 1,292,104 (1,829,258 ) — Other liabilities 22,838 29,317 1,949,696 (293,343 ) 1,708,508 Total liabilities 320,126 1,060,857 31,471,578 (11,046,838 ) 21,805,723 Redeemable noncontrolling interests — — 220,622 (14,700 ) 205,922 Shareholders’ Equity Total shareholders’ equity available to Arch 9,196,602 3,163,154 10,331,231 (13,494,385 ) 9,196,602 Non-redeemable noncontrolling interests — — 843,411 — 843,411 Total shareholders’ equity 9,196,602 3,163,154 11,174,642 (13,494,385 ) 10,040,013 Total liabilities, noncontrolling interests and shareholders’ equity $ 9,516,728 $ 4,224,011 $ 42,866,842 $ (24,555,923 ) $ 32,051,658
Condensed consolidating statement of income and comprehensive income	Three Months Ended June 30, 2018 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 1,336,763 $ — $ 1,336,763 Net investment income 15 560 157,532 (22,439 ) 135,668 Net realized gains (losses) — — (76,611 ) — (76,611 ) Net impairment losses recognized in earnings — — (470 ) — (470 ) Other underwriting income — — 3,874 — 3,874 Equity in net income (loss) of investment funds accounted for using the equity method — — 8,472 — 8,472 Other income (loss) 2,339 — 774 — 3,113 Total revenues 2,354 560 1,430,334 (22,439 ) 1,410,809 Expenses Losses and loss adjustment expenses — — 726,153 — 726,153 Acquisition expenses — — 202,838 — 202,838 Other operating expenses — — 176,181 — 176,181 Corporate expenses 16,642 470 5,400 — 22,512 Amortization of intangible assets — — 26,472 — 26,472 Interest expense 5,537 12,013 34,911 (22,117 ) 30,344 Net foreign exchange (gains) losses — — (43,357 ) (10,349 ) (53,706 ) Total expenses 22,179 12,483 1,128,598 (32,466 ) 1,130,794 Income (loss) before income taxes (19,825 ) (11,923 ) 301,736 10,027 280,015 Income tax (expense) benefit — 2,477 (26,145 ) — (23,668 ) Income (loss) before equity in net income of subsidiaries (19,825 ) (9,446 ) 275,591 10,027 256,347 Equity in net income of subsidiaries 263,471 86,727 — (350,198 ) — Net income 243,646 77,281 275,591 (340,171 ) 256,347 Net (income) loss attributable to noncontrolling interests — — (13,023 ) 322 (12,701 ) Net income available to Arch 243,646 77,281 262,568 (339,849 ) 243,646 Preferred dividends (10,403 ) — — — (10,403 ) Net income available to Arch common shareholders $ 233,243 $ 77,281 $ 262,568 $ (339,849 ) $ 233,243 Comprehensive income available to Arch $ 183,500 $ 70,066 $ 212,802 $ (282,868 ) $ 183,500 Three Months Ended June 30, 2017 Condensed Consolidating Statement of Income and Comprehensive Income Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Revenues Net premiums earned $ — $ — $ 1,240,874 $ — $ 1,240,874 Net investment income 1 184 133,627 (22,688 ) 111,124 Net realized gains (losses) — — 21,735 — 21,735 Net impairment losses recognized in earnings — — (1,730 ) — (1,730 ) Other underwriting income — — 4,822 — 4,822 Equity in net income (loss) of investment funds accounted for using the equity method — — 32,706 — 32,706 Other income (loss) (437 ) — (1,557 ) — (1,994 ) Total revenues (436 ) 184 1,430,477 (22,688 ) 1,407,537 Expenses Losses and loss adjustment expenses — — 689,860 — 689,860 Acquisition expenses — — 190,436 — 190,436 Other operating expenses — — 169,981 — 169,981 Corporate expenses 21,816 1,309 1,751 — 24,876 Amortization of intangible assets — — 30,824 — 30,824 Interest expense 6,075 11,989 33,050 (22,365 ) 28,749 Net foreign exchange (gains) losses — — 29,843 9,700 39,543 Total expenses 27,891 13,298 1,145,745 (12,665 ) 1,174,269 Income (loss) before income taxes (28,327 ) (13,114 ) 284,732 (10,023 ) 233,268 Income tax (expense) benefit — 4,069 (38,238 ) — (34,169 ) Income (loss) before equity in net income of subsidiaries (28,327 ) (9,045 ) 246,494 (10,023 ) 199,099 Equity in net income of subsidiaries 213,494 86,156 — (299,650 ) — Net income 185,167 77,111 246,494 (309,673 ) 199,099 Net (income) loss attributable to noncontrolling interests — — (14,254 ) 322 (13,932 ) Net income available to Arch 185,167 77,111 232,240 (309,351 ) 185,167 Preferred dividends (11,349 ) — — — (11,349 ) Net income available to Arch common shareholders $ 173,818 $ 77,111 $ 232,240 $ (309,351 ) $ 173,818 Comprehensive income available to Arch $ 279,285 $ 105,302 $ 475,747 $ (581,049 ) $ 279,285
Condensed consolidating statement of cash flows	Six Months Ended June 30, 2018 Condensed Consolidating Statement of Cash Flows Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Operating Activities Net Cash Provided By (Used For) Operating Activities $ 201,487 $ 149,576 $ 1,010,703 $ (864,880 ) $ 496,886 Investing Activities Purchases of fixed maturity investments — (125,440 ) (17,347,846 ) 605,716 (16,867,570 ) Purchases of equity securities — — (679,663 ) — (679,663 ) Purchases of other investments — — (1,017,147 ) — (1,017,147 ) Proceeds from the sales of fixed maturity investments — 33,793 16,662,466 (605,716 ) 16,090,543 Proceeds from the sales of equity securities — — 622,068 — 622,068 Proceeds from the sales, redemptions and maturities of other investments — — 773,298 — 773,298 Proceeds from redemptions and maturities of fixed maturity investments — — 511,448 — 511,448 Net settlements of derivative instruments — — 4,498 — 4,498 Net (purchases) sales of short-term investments 96,446 (59,798 ) 415,253 — 451,901 Change in cash collateral related to securities lending — — 176,304 — 176,304 Contributions to subsidiaries — (1,000 ) (22,595 ) 23,595 — Purchases of fixed assets (71 ) — (13,171 ) — (13,242 ) Other (4 ) — 49,965 — 49,961 Net Cash Provided By (Used For) Investing Activities 96,371 (152,445 ) 134,878 23,595 102,399 Financing Activities Redemption of preferred shares (92,555 ) — — — (92,555 ) Purchases of common shares under share repurchase program (173,575 ) — — — (173,575 ) Proceeds from common shares issued, net (13,851 ) — 23,595 (23,595 ) (13,851 ) Proceeds from borrowings — — 130,579 — 130,579 Repayments of borrowings — — (373,000 ) — (373,000 ) Change in cash collateral related to securities lending — — (176,304 ) — (176,304 ) Dividends paid to redeemable noncontrolling interests — — (9,632 ) 638 (8,994 ) Dividends paid to parent (1) — — (864,242 ) 864,242 — Other — — (4,489 ) — (4,489 ) Preferred dividends paid (20,840 ) — — — (20,840 ) Net Cash Provided By (Used For) Financing Activities (300,821 ) — (1,273,493 ) 841,285 (733,029 ) Effects of exchange rates changes on foreign currency cash and restricted cash — — (10,431 ) — (10,431 ) Increase (decrease) in cash and restricted cash (2,963 ) (2,869 ) (138,343 ) — (144,175 ) Cash and restricted cash, beginning of year 10,048 30,380 686,856 — 727,284 Cash and restricted cash, end of period $ 7,085 $ 27,511 $ 548,513 $ — $ 583,109 (1) Dividends received by parent are included in net cash provided by (used for) operating activities. Six Months Ended June 30, 2017 Condensed Consolidating Statement of Cash Flows Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Other Arch Capital Subsidiaries Consolidating Adjustments and Eliminations Arch Capital Consolidated Operating Activities Net Cash Provided By (Used For) Operating Activities $ 7,483 $ 27,853 $ 956,437 $ (458,238 ) $ 533,535 Investing Activities Purchases of fixed maturity investments — — (19,899,326 ) — (19,899,326 ) Purchases of equity securities — — (400,155 ) — (400,155 ) Purchases of other investments — — (883,704 ) — (883,704 ) Proceeds from the sales of fixed maturity investments — — 19,611,680 — 19,611,680 Proceeds from the sales of equity securities — — 473,064 — 473,064 Proceeds from the sales, redemptions and maturities of other investments — — 614,494 — 614,494 Proceeds from redemptions and maturities of fixed maturity investments — — 447,941 — 447,941 Net settlements of derivative instruments — — (5,984 ) — (5,984 ) Net (purchases) sales of short-term investments 2,354 (27,896 ) (419,661 ) — (445,203 ) Change in cash collateral related to securities lending — — 175,693 — 175,693 Contributions to subsidiaries 20,641 (72,900 ) (342,950 ) 395,209 — Issuance of intercompany loans — — (47,000 ) 47,000 — Repayment of intercompany loans — 47,000 — (47,000 ) — Purchases of fixed assets (18 ) (10 ) (11,075 ) — (11,103 ) Other — — 54,129 (20,641 ) 33,488 Net Cash Provided By (Used For) Investing Activities 22,977 (53,806 ) (632,854 ) 374,568 (289,115 ) Financing Activities Proceeds from common shares issued, net (6,838 ) — 395,209 (395,209 ) (6,838 ) Repayments of borrowings — — (72,000 ) — (72,000 ) Change in cash collateral related to securities lending — — (175,693 ) — (175,693 ) Dividends paid to redeemable noncontrolling interests — — (9,632 ) 638 (8,994 ) Dividends paid to parent (1) — — (457,600 ) 457,600 — Other — — (62,339 ) 20,641 (41,698 ) Preferred dividends paid (22,567 ) — — — (22,567 ) Net Cash Provided By (Used For) Financing Activities (29,405 ) — (382,055 ) 83,670 (327,790 ) Effects of exchange rates changes on foreign currency cash and restricted cash — — 9,616 — 9,616 Increase (decrease) in cash and restricted cash 1,055 (25,953 ) (48,856 ) — (73,754 ) Cash and restricted cash, beginning of year 1,738 71,955 895,876 — 969,569 Cash and restricted cash, end of period $ 2,793 $ 46,002 $ 847,020 $ — $ 895,815 (1) Dividends received by parent are included in net cash provided by (used for) operating activities.

Recent Accounting Pronounceme34

Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of an accounting change	$ 149,794	$ (314)
Retained earnings | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of an accounting change	149,800
Accumulated other comprehensive income (loss), net of deferred income tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of an accounting change	(149,794)	$ 0
Accumulated other comprehensive income (loss), net of deferred income tax | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of an accounting change	$ (149,800)

Share Transactions - Three-for-

Share Transactions - Three-for-one common share split (Details) shares in Millions	Jun. 18, 2018$ / sharesshares	Jun. 30, 2018$ / shares	Mar. 31, 2018$ / sharesshares	Dec. 31, 2017$ / shares
Stockholders' Equity Note [Abstract]
Share split conversion ratio	3
Number of common shares authorized | shares	1,800		600
Common shares, par value per share | $ / shares	$ 0.0011	$ 0.0011	$ 0.0033	$ 0.0011

Share Transactions - Share-base

Share Transactions - Share-based compensation (Details) - $ / shares	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted in period, shares	2,199,656	1,477,398
Options granted - Weighted average grant date fair value, per share	$ 7.43	$ 8.22
Restricted shares and units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards other than options granted in period, shares	1,264,931	1,534,908
Awards other than options granted - Weighted average grant date fair value, per share	$ 26.56	$ 32.09
Performance shares and units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards other than options granted in period, shares	705,345
Awards other than options granted - Weighted average grant date fair value, per share	$ 24.65

Share Transactions - Share repu

Share Transactions - Share repurchases (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018USD ($)shares
Class of Stock [Line Items]
Treasury stock, shares acquired (shares) | shares	6,522,645
Shares repurchased for treasury	$ 173.6
Common shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program | shares	382,200,000
Aggregate purchase price of shares acquired since inception of share repurchase program	$ 3,860
Remaining authorized repurchase amount	$ 272.9

Share Transactions - Conversion

Share Transactions - Conversion of Non-voting common equivalent preferred shares (Details)	1 Months Ended
	Mar. 31, 2018shares
Class of Stock [Line Items]
Number of common shares issued on conversion	17,022,600
Number of convertible shares transferred	567,420

Share Transactions - Series C P

Share Transactions - Series C Preferred Shares (Details) - Series C Preferred Shares	Jan. 02, 2018$ / shares
Class of Stock [Line Items]
Preferred shares, dividend rate	6.75%
Preferred shares, redemption price	$ 25

Earnings Per Common Share (Deta

Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income		$ 256,347	$ 199,099	$ 422,731	$ 473,134
Net (income) loss attributable to noncontrolling interests		(12,701)	(13,932)	(28,662)	(34,840)
Net income available to Arch		243,646	185,167	394,069	438,294
Preferred dividends		(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares		0	0	(2,710)	0
Net income available to Arch common shareholders		$ 233,243	$ 173,818	$ 370,519	$ 415,727
Denominator:
Weighted average common shares outstanding		404,800,421	369,027,702	399,485,135	366,436,407
Series D preferred shares		0	34,432,290	6,677,373	36,349,722
Weighted average common shares outstanding — basic		404,800,421	403,459,992	406,162,508	402,786,129
Effect of dilutive common share equivalents:
Nonvested restricted shares		1,575,749	4,448,889	1,837,356	4,670,889
Stock options	[1]	6,735,035	9,825,057	7,460,892	9,964,878
Weighted average common shares and common share equivalents outstanding — diluted		413,111,205	417,733,938	415,460,756	417,421,896
Earnings per common share:
Basic (per share)		$ 0.58	$ 0.43	$ 0.91	$ 1.03
Diluted (per share)		$ 0.56	$ 0.42	$ 0.89	$ 1
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)		5,350,733	1,499,997	5,372,789	2,292,228
[1]	Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For the 2018 second quarter and 2017 second quarter, the number of stock options excluded were 5,350,733 and 1,499,997, respectively. For the six months ended June 30, 2018 and 2017, the number of stock options excluded were 5,372,789 and 2,292,228, respectively.

Segment Information - Summary o

Segment Information - Summary of underwriting income or loss by segment (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,696,544		$ 1,609,659	$ 3,534,758		$ 3,267,649
Premiums ceded		(397,648)		(360,964)	(823,318)		(742,694)
Net premiums written		1,298,896		1,248,695	2,711,440		2,524,955
Change in unearned premiums		37,867		(7,821)	(139,778)		(167,064)
Net premiums earned		1,336,763		1,240,874	2,571,662		2,357,891
Other underwriting income		3,874		4,822	9,223		9,455
Losses and loss adjustment expenses		(726,153)		(689,860)	(1,363,013)		(1,242,430)
Acquisition expenses, net		(202,838)		(190,436)	(394,214)		(372,725)
Other operating expenses		(176,181)		(169,981)	(351,196)		(344,700)
Underwriting income (loss)		235,465		195,419	472,462		407,491
Net investment income		135,668		111,124	262,392		228,998
Net realized gains (losses)		(76,611)		21,735	(187,609)		55,888
Net impairment losses recognized in earnings		(470)		(1,730)	(632)		(3,537)
Equity in net income (loss) of investment funds accounted for using the equity method		8,472		32,706	36,541		80,794
Other income (loss)		3,113		(1,994)	3,187		(2,776)
Corporate expenses		(15,604)	[1]	(22,201)	(30,086)	[1]	(34,409)	[1]
UGC transaction costs and other		(6,908)	[1]	(2,675)	(7,738)	[1]	(18,259)	[1]
Amortization of intangible assets		(26,472)		(30,824)	(53,208)		(62,118)
Interest expense		(30,344)		(28,749)	(60,980)		(57,425)
Net foreign exchange gains (losses)		53,706		(39,543)	33,985		(58,947)
Income before income taxes		280,015		233,268	468,314		535,700
Income tax expense		(23,668)		(34,169)	(45,583)		(62,566)
Net income (loss)		256,347		199,099	422,731		473,134
Dividends attributable to redeemable noncontrolling interests		(4,585)		(4,586)	(9,170)		(9,170)
Amounts attributable to non-redeemable noncontrolling interests		(8,116)		(9,346)	(19,492)		(25,670)
Net income available to Arch		243,646		185,167	394,069		438,294
Preferred dividends		(10,403)		(11,349)	(20,840)		(22,567)
Loss on redemption of preferred shares		0		0	(2,710)		0
Net income available to Arch common shareholders		$ 233,243		$ 173,818	$ 370,519		$ 415,727
Underwriting Ratios
Loss ratio		54.30%		55.60%	53.00%		52.70%
Acquisition expense ratio		15.20%		15.30%	15.30%		15.80%
Other operating expense ratio		13.20%		13.70%	13.70%		14.60%
Combined ratio		82.70%		84.60%	82.00%		83.10%
Goodwill and intangible assets		$ 593,008		$ 712,975	$ 593,008		$ 712,975		$ 652,611
Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	175,175		152,813	389,045		306,933
Premiums ceded		(34,589)		(12,410)	(68,907)		(22,844)
Net premiums written		140,586		140,403	320,138		284,089
Change in unearned premiums		18,932		10,351	(23,872)		(11,338)
Net premiums earned		159,518		150,754	296,266		272,751
Other underwriting income		688		824	1,389		1,640
Losses and loss adjustment expenses		(117,141)		(110,621)	(215,130)		(196,031)
Acquisition expenses, net		(34,289)		(34,747)	(65,610)		(67,255)
Other operating expenses		(9,094)		(8,853)	(17,793)		(16,043)
Underwriting income (loss)		(318)		(2,643)	(878)		(4,938)
Net investment income		27,907		18,604	54,388		40,666
Net realized gains (losses)		(17,066)		3,689	(16,205)		9,330
Net impairment losses recognized in earnings		0		0	0		0
Equity in net income (loss) of investment funds accounted for using the equity method		0		0	0		0
Other income (loss)		0		0	0		0
Corporate expenses		0	[1]	0	0	[1]	0	[1]
UGC transaction costs and other		0	[1]	0	0	[1]	0	[1]
Amortization of intangible assets		0		0	0		0
Interest expense		(4,286)		(2,837)	(9,015)		(5,757)
Net foreign exchange gains (losses)		7,495		(1,722)	2,813		(1,281)
Income before income taxes		13,732		15,091	31,103		38,020
Income tax expense		(24)		0	(27)		0
Net income (loss)		13,708		15,091	31,076		38,020
Dividends attributable to redeemable noncontrolling interests		(4,585)		(4,586)	(9,170)		(9,170)
Amounts attributable to non-redeemable noncontrolling interests		(8,116)		(9,346)	(19,492)		(25,670)
Net income available to Arch		1,007		1,159	2,414		3,180
Preferred dividends		0		0	0		0
Loss on redemption of preferred shares					0
Net income available to Arch common shareholders		$ 1,007		$ 1,159	$ 2,414		$ 3,180
Underwriting Ratios
Loss ratio		73.40%		73.40%	72.60%		71.90%
Acquisition expense ratio		21.50%		23.00%	22.10%		24.70%
Other operating expense ratio		5.70%		5.90%	6.00%		5.90%
Combined ratio		100.60%		102.30%	100.70%		102.50%
Goodwill and intangible assets		$ 7,650		$ 7,650	$ 7,650		$ 7,650
Sub-Total
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,591,202		1,533,142	3,312,807		3,139,828
Premiums ceded		(432,892)		(424,850)	(921,505)		(898,962)
Net premiums written		1,158,310		1,108,292	2,391,302		2,240,866
Change in unearned premiums		18,935		(18,172)	(115,906)		(155,726)
Net premiums earned		1,177,245		1,090,120	2,275,396		2,085,140
Other underwriting income		3,186		3,998	7,834		7,815
Losses and loss adjustment expenses		(609,012)		(579,239)	(1,147,883)		(1,046,399)
Acquisition expenses, net		(168,549)		(155,689)	(328,604)		(305,470)
Other operating expenses		(167,087)		(161,128)	(333,403)		(328,657)
Underwriting income (loss)		235,783		198,062	473,340		412,429
Net investment income		107,761		92,520	208,004		188,332
Net realized gains (losses)		(59,545)		18,046	(171,404)		46,558
Net impairment losses recognized in earnings		(470)		(1,730)	(632)		(3,537)
Equity in net income (loss) of investment funds accounted for using the equity method		8,472		32,706	36,541		80,794
Other income (loss)		3,113		(1,994)	3,187		(2,776)
Corporate expenses		(15,604)	[1]	(22,201)	(30,086)	[1]	(34,409)	[1]
UGC transaction costs and other		(6,908)	[1]	(2,675)	(7,738)	[1]	(18,259)	[1]
Amortization of intangible assets		(26,472)		(30,824)	(53,208)		(62,118)
Interest expense		(26,058)		(25,912)	(51,965)		(51,668)
Net foreign exchange gains (losses)		46,211		(37,821)	31,172		(57,666)
Income before income taxes		266,283		218,177	437,211		497,680
Income tax expense		(23,644)		(34,169)	(45,556)		(62,566)
Net income (loss)		242,639		184,008	391,655		435,114
Dividends attributable to redeemable noncontrolling interests		0		0	0		0
Amounts attributable to non-redeemable noncontrolling interests		0		0	0		0
Net income available to Arch		242,639		184,008	391,655		435,114
Preferred dividends		(10,403)		(11,349)	(20,840)		(22,567)
Loss on redemption of preferred shares					(2,710)
Net income available to Arch common shareholders		$ 232,236		$ 172,659	$ 368,105		$ 412,547
Underwriting Ratios
Loss ratio		51.70%		53.10%	50.40%		50.20%
Acquisition expense ratio		14.30%		14.30%	14.40%		14.60%
Other operating expense ratio		14.20%		14.80%	14.70%		15.80%
Combined ratio		80.20%		82.20%	79.50%		80.60%
Goodwill and intangible assets		$ 585,358		$ 705,325	$ 585,358		$ 705,325
Sub-Total | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	769,372		743,902	1,592,750		1,526,183
Premiums ceded		(245,265)		(247,446)	(492,445)		(481,541)
Net premiums written		524,107		496,456	1,100,305		1,044,642
Change in unearned premiums		22,342		21,118	(15,119)		(21,422)
Net premiums earned		546,449		517,574	1,085,186		1,023,220
Other underwriting income		0		0	0		0
Losses and loss adjustment expenses		(357,465)		(350,939)	(711,195)		(683,580)
Acquisition expenses, net		(90,670)		(78,872)	(175,839)		(153,740)
Other operating expenses		(92,680)		(92,267)	(184,654)		(180,393)
Underwriting income (loss)		$ 5,634		$ (4,504)	$ 13,498		$ 5,507
Underwriting Ratios
Loss ratio		65.40%		67.80%	65.50%		66.80%
Acquisition expense ratio		16.60%		15.20%	16.20%		15.00%
Other operating expense ratio		17.00%		17.80%	17.00%		17.60%
Combined ratio		99.00%		100.80%	98.70%		99.40%
Goodwill and intangible assets		$ 20,724		$ 24,480	$ 20,724		$ 24,480
Sub-Total | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	490,327		453,186	1,067,810		928,968
Premiums ceded		(136,247)		(115,262)	(331,977)		(281,354)
Net premiums written		354,080		337,924	735,833		647,614
Change in unearned premiums		(13,762)		(23,222)	(116,343)		(88,061)
Net premiums earned		340,318		314,702	619,490		559,553
Other underwriting income		(129)		(279)	1,103		(585)
Losses and loss adjustment expenses		(229,956)		(207,606)	(371,631)		(313,060)
Acquisition expenses, net		(50,142)		(51,151)	(98,461)		(97,298)
Other operating expenses		(35,678)		(36,711)	(71,249)		(74,244)
Underwriting income (loss)		$ 24,413		$ 18,955	$ 79,252		$ 74,366
Underwriting Ratios
Loss ratio		67.60%		66.00%	60.00%		55.90%
Acquisition expense ratio		14.70%		16.30%	15.90%		17.40%
Other operating expense ratio		10.50%		11.70%	11.50%		13.30%
Combined ratio		92.80%		94.00%	87.40%		86.60%
Goodwill and intangible assets		$ 0		$ 609	$ 0		$ 609
Sub-Total | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[2]	330,990		336,226	652,168		684,849
Premiums ceded		(50,867)		(62,314)	(97,004)		(136,239)
Net premiums written		280,123		273,912	555,164		548,610
Change in unearned premiums		10,355		(16,068)	15,556		(46,243)
Net premiums earned		290,478		257,844	570,720		502,367
Other underwriting income		3,315		4,277	6,731		8,400
Losses and loss adjustment expenses		(21,591)		(20,694)	(65,057)		(49,759)
Acquisition expenses, net		(27,737)		(25,666)	(54,304)		(54,432)
Other operating expenses		(38,729)		(32,150)	(77,500)		(74,020)
Underwriting income (loss)		$ 205,736		$ 183,611	$ 380,590		$ 332,556
Underwriting Ratios
Loss ratio		7.40%		8.00%	11.40%		9.90%
Acquisition expense ratio		9.50%		10.00%	9.50%		10.80%
Other operating expense ratio		13.30%		12.50%	13.60%		14.70%
Combined ratio		30.20%		30.50%	34.50%		35.40%
Goodwill and intangible assets		$ 564,634		$ 680,236	$ 564,634		$ 680,236
[1]	Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘UGC transaction costs and other.’
[2]	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

Segment Information - Narrative

Segment Information - Narrative (Details)	6 Months Ended
	Jun. 30, 2018segment
Underwriting segments
Segment Reporting Information [Line Items]
Number of segments	3
Other
Segment Reporting Information [Line Items]
Number of segments	2

Reserve for Losses and Loss A43

Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves(Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 11,496,205	$ 10,296,821	$ 11,383,792	$ 10,200,960
Unpaid losses and loss adjustment expenses recoverable		2,446,990	2,095,130	2,464,910	2,083,575
Net reserve for losses and loss adjustment expenses at beginning of period		9,049,215	8,201,691	8,918,882	8,117,385
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		790,742	759,261	1,478,627	1,395,037
Prior years		(64,589)	(69,401)	(115,614)	(152,607)
Total net incurred losses and loss adjustment expenses		726,153	689,860	1,363,013	1,242,430
Retroactive reinsurance transaction	[1]	(420,404)	0	(420,404)	0
Net foreign exchange (gains) losses		(118,542)	75,295	(74,528)	106,574
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(59,022)	(80,499)	(95,022)	(115,502)
Prior years		(404,812)	(482,046)	(919,353)	(946,586)
Total net paid losses and loss adjustment expenses		(463,834)	(562,545)	(1,014,375)	(1,062,088)
Net reserve for losses and loss adjustment expenses at end of period		8,772,588	8,404,301	8,772,588	8,404,301
Unpaid losses and loss adjustment expenses recoverable		2,651,749	2,116,210	2,651,749	2,116,210
Reserve for losses and loss adjustment expenses at end of period		$ 11,424,337	$ 10,520,511	$ 11,424,337	$ 10,520,511
[1]	During the 2018 second quarter, a subsidiary of the Company entered into a retroactive reinsurance transaction with a third party reinsurer to reinsure run-off liabilities associated with certain discontinued U.S. specialty casualty and program exposures.

Reserve for Losses and Loss A44

Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 64,589	$ 69,401	$ 115,614	$ 152,607
Underwriting segments | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 33,000	$ 39,500	$ 69,600	$ 96,800
Percentage of prior year development	9.70%	12.60%	11.20%	17.30%
Underwriting segments | Reinsurance | Short Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 22,200	$ 28,100	$ 51,100	$ 68,900
Underwriting segments | Reinsurance | Short Tailed Lines | Property catastrophe and property other than property catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	19,300	16,900	40,400	51,000
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	10,800	11,400	18,500	27,900
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	6,900	9,000	8,100	15,600
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	3,800	2,400	10,000	12,300
Underwriting segments | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 6,100	$ 2,000	$ 8,200	$ 4,100
Percentage of prior year development	1.10%	0.40%	0.80%	0.40%
Underwriting segments | Insurance | Short Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 13,900	$ 5,300	$ 22,700	$ 7,200
Underwriting segments | Insurance | Medium Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(22,100)		(31,700)	(9,700)
Underwriting segments | Insurance | Medium Tailed Lines | Professional Liability And Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years				16,700
Underwriting segments | Insurance | Medium Tailed Lines | Professional lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		12,100
Underwriting segments | Insurance | Medium Tailed Lines | Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		3,600
Underwriting segments | Insurance | Medium Tailed Lines | Programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(11,600)		(21,900)	(26,400)
Underwriting segments | Insurance | Medium Tailed Lines | Contract binding
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(18,000)		(25,600)
Underwriting segments | Insurance | Medium Tailed Lines | Other product lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	7,600	17,800	15,800
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(3,300)
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines | Programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(12,200)
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines | Construction
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(8,900)
Underwriting segments | Insurance | Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	14,300		17,200	6,600
Underwriting segments | Insurance | Long Tailed Lines | Executive assurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	6,900		7,500
Underwriting segments | Insurance | Long Tailed Lines | Healthcare
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	4,900		7,000
Underwriting segments | Insurance | Long Tailed Lines | Construction
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years				(13,400)
Underwriting segments | Mortgage
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 23,300	$ 29,800	$ 36,300	$ 53,400
Percentage of prior year development	8.00%	11.50%	6.40%	10.60%
Underwriting segments | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 2,200	$ 1,900	$ 1,600	$ (1,700)

Investment Information - Summar

Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Available for sale securities:
Estimated Fair Value		$ 15,444,258		$ 16,570,755
Gross Unrealized Gains		44,033		267,433
Gross Unrealized Losses		(207,692)		(112,447)
Cost or Amortized Cost		15,607,917		16,415,769
OTTI Unrealized Losses		(84)	[1]	(88)
Fixed maturities
Available for sale securities:
Estimated Fair Value		14,347,460		14,332,391
Gross Unrealized Gains	[2]	43,695		111,098
Gross Unrealized Losses	[2]	(207,620)		(106,181)
Cost or Amortized Cost	[2]	14,511,385		14,327,474
OTTI Unrealized Losses	[1],[2]	(84)		(88)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value		5,535,763		4,434,439
Gross Unrealized Gains	[2]	7,199		30,943
Gross Unrealized Losses	[2]	(103,978)		(32,340)
Cost or Amortized Cost	[2]	5,632,542		4,435,836
OTTI Unrealized Losses	[1],[2]	(69)		(73)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value		488,699		316,141
Gross Unrealized Gains	[2]	1,699		1,640
Gross Unrealized Losses	[2]	(4,742)		(2,561)
Cost or Amortized Cost	[2]	491,742		317,062
OTTI Unrealized Losses	[1],[2]	(15)		(15)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value		1,431,256		2,158,840
Gross Unrealized Gains	[2]	7,371		20,285
Gross Unrealized Losses	[2]	(22,047)		(12,308)
Cost or Amortized Cost	[2]	1,445,932		2,150,863
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value		590,198		545,817
Gross Unrealized Gains	[2]	776		2,131
Gross Unrealized Losses	[2]	(13,222)		(4,268)
Cost or Amortized Cost	[2]	602,644		547,954
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value		2,788,272		3,484,257
Gross Unrealized Gains	[2]	6,216		2,188
Gross Unrealized Losses	[2]	(19,577)		(28,769)
Cost or Amortized Cost	[2]	2,801,633		3,510,838
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value		1,692,783		1,612,754
Gross Unrealized Gains	[2]	17,124		48,764
Gross Unrealized Losses	[2]	(27,480)		(17,321)
Cost or Amortized Cost	[2]	1,703,139		1,581,311
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value		1,820,489		1,780,143
Gross Unrealized Gains	[2]	3,310		5,147
Gross Unrealized Losses	[2]	(16,574)		(8,614)
Cost or Amortized Cost	[2]	1,833,753		1,783,610
OTTI Unrealized Losses	[1],[2]	0		0
Equity securities
Available for sale securities:
Estimated Fair Value	[3]			504,333
Gross Unrealized Gains	[3]			88,739
Gross Unrealized Losses	[3]			(5,583)
Cost or Amortized Cost	[3]			421,177
OTTI Unrealized Losses	[1]			0
Other investments
Available for sale securities:
Estimated Fair Value		0		264,989
Gross Unrealized Gains		0		66,946
Gross Unrealized Losses		0		(120)
Cost or Amortized Cost		0		198,163
OTTI Unrealized Losses	[1]	0		0
Short-term investments
Available for sale securities:
Estimated Fair Value		1,096,798		1,469,042
Gross Unrealized Gains		338		650
Gross Unrealized Losses		(72)		(563)
Cost or Amortized Cost		1,096,532		1,468,955
OTTI Unrealized Losses	[1]	$ 0		$ 0
[1]	Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million, compared to a net unrealized gain of $0.3 million at December 31, 2017.
[2]	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”
[3]	Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table.

Investment Information - Aging

Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Available for sale securities:
Estimated Fair Value - Less than 12 Months		$ 10,635,259	$ 9,064,672
Gross Unrealized Losses - Less than 12 Months		(181,808)	(91,691)
Estimated Fair Value - 12 Months or More		715,345	911,838
Gross Unrealized Losses - 12 Months or More		(25,884)	(20,756)
Estimated Fair Value - Total		11,350,604	9,976,510
Gross Unrealized Losses - Total		(207,692)	(112,447)
Fixed maturities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	10,581,322	8,773,557
Gross Unrealized Losses - Less than 12 Months	[1]	(181,736)	(85,425)
Estimated Fair Value - 12 Months or More	[1]	715,345	911,838
Gross Unrealized Losses - 12 Months or More	[1]	(25,884)	(20,756)
Estimated Fair Value - Total	[1]	11,296,667	9,685,395
Gross Unrealized Losses - Total	[1]	(207,620)	(106,181)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	4,541,077	2,320,716
Gross Unrealized Losses - Less than 12 Months	[1]	(94,563)	(25,411)
Estimated Fair Value - 12 Months or More	[1]	236,332	279,082
Gross Unrealized Losses - 12 Months or More	[1]	(9,415)	(6,929)
Estimated Fair Value - Total	[1]	4,777,409	2,599,798
Gross Unrealized Losses - Total	[1]	(103,978)	(32,340)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	296,876	221,113
Gross Unrealized Losses - Less than 12 Months	[1]	(4,618)	(1,715)
Estimated Fair Value - 12 Months or More	[1]	708	28,380
Gross Unrealized Losses - 12 Months or More	[1]	(124)	(846)
Estimated Fair Value - Total	[1]	297,584	249,493
Gross Unrealized Losses - Total	[1]	(4,742)	(2,561)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	814,694	1,030,389
Gross Unrealized Losses - Less than 12 Months	[1]	(16,888)	(8,438)
Estimated Fair Value - 12 Months or More	[1]	109,913	132,469
Gross Unrealized Losses - 12 Months or More	[1]	(5,159)	(3,870)
Estimated Fair Value - Total	[1]	924,607	1,162,858
Gross Unrealized Losses - Total	[1]	(22,047)	(12,308)
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	351,013	225,164
Gross Unrealized Losses - Less than 12 Months	[1]	(9,067)	(1,899)
Estimated Fair Value - 12 Months or More	[1]	56,382	57,291
Gross Unrealized Losses - 12 Months or More	[1]	(4,155)	(2,369)
Estimated Fair Value - Total	[1]	407,395	282,455
Gross Unrealized Losses - Total	[1]	(13,222)	(4,268)
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,957,338	2,646,415
Gross Unrealized Losses - Less than 12 Months	[1]	(18,470)	(26,501)
Estimated Fair Value - 12 Months or More	[1]	41,069	111,879
Gross Unrealized Losses - 12 Months or More	[1]	(1,107)	(2,268)
Estimated Fair Value - Total	[1]	1,998,407	2,758,294
Gross Unrealized Losses - Total	[1]	(19,577)	(28,769)
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,243,055	1,218,514
Gross Unrealized Losses - Less than 12 Months	[1]	(24,034)	(15,546)
Estimated Fair Value - 12 Months or More	[1]	160,434	93,530
Gross Unrealized Losses - 12 Months or More	[1]	(3,446)	(1,775)
Estimated Fair Value - Total	[1]	1,403,489	1,312,044
Gross Unrealized Losses - Total	[1]	(27,480)	(17,321)
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,377,269	1,111,246
Gross Unrealized Losses - Less than 12 Months	[1]	(14,096)	(5,915)
Estimated Fair Value - 12 Months or More	[1]	110,507	209,207
Gross Unrealized Losses - 12 Months or More	[1]	(2,478)	(2,699)
Estimated Fair Value - Total	[1]	1,487,776	1,320,453
Gross Unrealized Losses - Total	[1]	(16,574)	(8,614)
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[2]		166,562
Gross Unrealized Losses - Less than 12 Months	[2]		(5,583)
Estimated Fair Value - 12 Months or More	[2]		0
Gross Unrealized Losses - 12 Months or More	[2]		0
Estimated Fair Value - Total	[2]		166,562
Gross Unrealized Losses - Total	[2]		(5,583)
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		0	15,025
Gross Unrealized Losses - Less than 12 Months		0	(120)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		0	15,025
Gross Unrealized Losses - Total		0	(120)
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		53,937	109,528
Gross Unrealized Losses - Less than 12 Months		(72)	(563)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		53,937	109,528
Gross Unrealized Losses - Total		$ (72)	$ (563)
[1]	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”
[2]	Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table.

Investment Information - Maturi

Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Estimated Fair Value:
Estimated fair value		$ 14,128,989	$ 13,876,003
Amortized Cost:
Amortized cost		14,293,121	13,869,460
Fixed maturities
Estimated Fair Value:
Due in one year or less		344,007	550,711
Due after one year through five years		8,154,773	7,436,153
Due after five years through 10 years		2,703,341	3,369,635
Due after 10 years		245,953	333,791
Single maturity date		11,448,074	11,690,290
Estimated fair value	[1]	14,347,460	14,332,391
Amortized Cost:
Due in one year or less		343,938	548,771
Due after one year through five years		8,235,481	7,434,801
Due after five years through 10 years		2,756,097	3,369,750
Due after 10 years		247,730	325,526
Single maturity date		11,583,246	11,678,848
Amortized cost	[1]	14,511,385	14,327,474
Fixed maturities | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		488,699	316,141
Amortized Cost:
Securities without single maturity date		491,742	317,062
Fixed maturities | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		590,198	545,817
Amortized Cost:
Securities without single maturity date		602,644	547,954
Fixed maturities | Asset backed securities
Estimated Fair Value:
Securities without single maturity date		1,820,489	1,780,143
Amortized Cost:
Securities without single maturity date		$ 1,833,753	$ 1,783,610
[1]	In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”

Investment Information - Securi

Investment Information - Securities lending agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Disclosure Investment Information [Abstract]
Fair value of cash collateral received on securities lending	$ 28,000	$ 199,900
Fair value of non-cash collateral received on securities lending	209,000	276,700
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	236,948	476,605
Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 9	0	0
Amounts related to securities lending not included in offsetting disclosure in Note 9	236,948	476,605
US government and government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	195,777	439,820
Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	29,265	28,003
Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	11,906	8,782
Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	188,680	380,210
Overnight and continuous | US government and government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	147,509	343,425
Overnight and continuous | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	29,265	28,003
Overnight and continuous | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	11,906	8,782
Less than 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	20,309
Less than 30 days | US government and government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	20,309
Less than 30 days | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
Less than 30 days | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
30 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	48,268	76,086
30 - 90 days | US government and government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	48,268	76,086
30 - 90 days | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
30 - 90 days | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
90 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
90 days or more | US government and government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
90 days or more | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	0	0
90 days or more | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending payable	$ 0	$ 0

Investment Information - Other

Investment Information - Other investments (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		$ 2,328,904	$ 2,401,370
Available for sale
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	[1]	0	264,989
Available for sale | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	135,140
Available for sale | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	53,878
Available for sale | Credit related funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	18,365
Available for sale | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		0	57,606
Fair value option
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		2,328,904	2,136,381
Fair value option | Term loan investments
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		1,286,305	1,200,882
Other investments par		1,321,154	1,223,453
Fair value option | Mezzanine debt funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		257,750	252,160
Fair value option | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		350,308	258,541
Fair value option | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		98,240	102,347
Fair value option | Credit related funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments		199,437	175,422
Fair value option | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	[2]	$ 136,864	$ 147,029
[1]	The Company reviewed the accounting treatment for three limited partnership investments which were accounted for as available for sale at December 31, 2017 during the 2018 first quarter and determined, based on reconsideration during the period of the Company’s percentage ownership, that the equity method of accounting was appropriate for such investments.
[2]	Includes fund investments with strategies in mortgage servicing rights, transportation, infrastructure assets and other.

Investment Information - Fair v

Investment Information - Fair value option (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$ 4,111,611	$ 4,216,237
Fixed maturities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	1,428,503	1,642,855
Other investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	2,328,904	2,136,381
Short-term investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	217,066	297,426
Equity securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$ 137,138	$ 139,575

Investment Information - Limite

Investment Information - Limited partnership interests (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss		$ 1,423,044	$ 1,171,792
Aggregate unfunded commitments		1,630,000	1,700,000
Equity method investments
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss	[1]	1,292,573	1,041,321
Aggregate unfunded commitments		994,700	1,020,000
Fair value option
Schedule of Equity Method Investments [Line Items]
Maximum Exposure to Loss	[2]	130,471	130,471
Aggregate unfunded commitments		$ 91,000	$ 100,400
[1]	Aggregate unfunded commitments were $994.7 million at June 30, 2018, compared to $1.02 billion at December 31, 2017.
[2]	Aggregate unfunded commitments were $91.0 million at June 30, 2018, compared to $100.4 million at December 31, 2017.

Investment Information - Net in

Investment Information - Net investment income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net investment income:
Gross investment income	$ 162,447	$ 134,016	$ 315,136	$ 272,391
Investment expenses	(26,779)	(22,892)	(52,744)	(43,393)
Net investment income	135,668	111,124	262,392	228,998
Fixed maturities
Net investment income:
Gross investment income	115,110	94,270	222,997	188,663
Equity securities
Net investment income:
Gross investment income	4,777	3,654	7,345	6,297
Short-term investments
Net investment income:
Gross investment income	4,392	2,016	9,252	3,775
Other
Net investment income:
Gross investment income	$ 38,168	$ 34,076	$ 75,542	$ 73,656

Investment Information - Net re

Investment Information - Net realized gains and losses (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net realized gains and losses:
Available for sale securities, gross gains on investment sales		$ 18,777	$ 76,730	$ 33,742	$ 145,905
Available for sale securities, gross losses on investment sales		(57,711)	(52,619)	(140,262)	(113,981)
Net realized gains (losses) on sales during the period		(5,918)	0	(11,286)	0
Net unrealized gains (losses) on equity securities still held at reporting date		(7,278)	0	(14,861)	0
Derivative instruments	[1]	(2,146)	(4,770)	(6,109)	(13,951)
Other	[2]	(248)	(14,056)	(9,338)	(19,873)
Net realized gains (losses)		(76,611)	21,735	(187,609)	55,888
Fixed maturities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		(22,927)	9,656	(40,478)	30,197
Other investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		(254)	637	(6,628)	17,885
Equity securities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		1,230	2,829	7,898	6,374
Short-term investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option		$ (136)	$ 3,328	$ (287)	$ 3,332
[1]	See Note 9 for information on the Company’s derivative instruments.
[2]	Includes the re-measurement of contingent consideration liability amounts.

Investment Information - Net im

Investment Information - Net impairment losses recognized in earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	$ (470)	$ (1,730)	$ (632)	$ (3,537)
Fixed maturities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(470)	(1,666)	(632)	(3,184)
Fixed maturities | Mortgage backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(81)	(92)	(123)	(1,411)
Fixed maturities | Corporate bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(241)	(1,401)	(361)	(1,402)
Fixed maturities | Non-US government securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings			0	(198)
Fixed maturities | Asset backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	(148)	0	(148)	0
Fixed maturities | Municipal bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	0	(173)	0	(173)
Equity securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings			0	(186)
Other investments
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	$ 0	$ (64)	$ 0	$ (167)

Investment Information - Othe55

Investment Information - Other than temporary impairments rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Rollforward:
Balance at start of period	$ 767	$ 12,537	$ 767	$ 13,138
Credit loss impairments recognized on securities not previously impaired	0	31	0	31
Credit loss impairments recognized on securities previously impaired	0	172	0	195
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	0	0	0	0
Reductions for securities sold during the period	(69)	(8,303)	(69)	(8,927)
Balance at end of period	$ 698	$ 4,437	$ 698	$ 4,437

Investment Information - Restri

Investment Information - Restricted assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Restricted Assets [Line Items]
Restricted assets	$ 6,845,855	$ 6,670,912
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,549,122	4,323,726
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	1,537,336	1,674,304
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	701,026	616,987
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	$ 58,371	$ 55,895

Investment Information - Narrat

Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2018USD ($)lots	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)lots		Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)lots	Mar. 31, 2018companies
Narrative items:
Number of Limited Partnerships | companies							3
Investable assets	$ 21,820,000		$ 21,820,000
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	$ 32,706	36,541		$ 80,794
OTTI unrealized (losses) gains at current fair value	$ 200		$ 200			$ 300
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	5,440		5,440			3,830
Total number of positions (lots) | lots	7,450		7,450			7,450
Largest single loss	$ 1,700		$ 1,700			$ 1,300
OTTI unrealized losses (gains)			$ 84	[1]		$ 88
Minimum
Narrative items:
Time lag for reporting			1 month
Maximum
Narrative items:
Time lag for reporting			3 months
Underwriting segments
Narrative items:
Investable assets	19,170,000		$ 19,170,000
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541		80,794
Other
Narrative items:
Investable assets	2,650,000		2,650,000
Equity in net income (loss) of investment funds accounted for using the equity method	$ 0	$ 0	$ 0		$ 0
[1]	Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million, compared to a net unrealized gain of $0.3 million at December 31, 2017.

Fair Value - Fair Value Hierarc

Fair Value - Fair Value Hierarchy (Details) - Recurring - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		$ 4,479,066	$ 5,823,343
Liabilities measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	[1]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	2,660,621	3,408,902
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	2,660,621	3,408,902
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,053,661	1,420,732
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		493,975	498,182
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			74,611
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			74,611
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		270,809	420,916
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		119,875	231,019
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		19,869	40,166
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		65,400	67,440
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		65,665	82,291
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		14,507,544	13,695,776
Liabilities measured at fair value		(46,927)	(54,839)
Significant Other Observable Inputs (Level 2) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		0	0
Significant Other Observable Inputs (Level 2) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	[1]	(24,529)	(34,375)
Significant Other Observable Inputs (Level 2) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		(22,398)	(20,464)
Significant Other Observable Inputs (Level 2) | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	11,677,690	10,908,102
Significant Other Observable Inputs (Level 2) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	5,526,990	4,424,979
Significant Other Observable Inputs (Level 2) | Fixed maturities | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	488,352	315,754
Significant Other Observable Inputs (Level 2) | Fixed maturities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	1,431,256	2,158,840
Significant Other Observable Inputs (Level 2) | Fixed maturities | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	590,169	545,277
Significant Other Observable Inputs (Level 2) | Fixed maturities | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	127,651	75,355
Significant Other Observable Inputs (Level 2) | Fixed maturities | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	1,692,783	1,612,754
Significant Other Observable Inputs (Level 2) | Fixed maturities | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	1,820,489	1,775,143
Significant Other Observable Inputs (Level 2) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		43,137	48,310
Significant Other Observable Inputs (Level 2) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		52,100	6,151
Significant Other Observable Inputs (Level 2) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			1,816
Significant Other Observable Inputs (Level 2) | Other investments at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			1,816
Significant Other Observable Inputs (Level 2) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		31,623	15,747
Significant Other Observable Inputs (Level 2) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		2,702,994	2,715,650
Significant Other Observable Inputs (Level 2) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		925,505	1,056,508
Significant Other Observable Inputs (Level 2) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		16,410	20,491
Significant Other Observable Inputs (Level 2) | Fair value option | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		7,082	15,210
Significant Other Observable Inputs (Level 2) | Fair value option | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,306	11,997
Significant Other Observable Inputs (Level 2) | Fair value option | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		110	271
Significant Other Observable Inputs (Level 2) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		132,162	195,788
Significant Other Observable Inputs (Level 2) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		214,718	99,354
Significant Other Observable Inputs (Level 2) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		197,197	257,260
Significant Other Observable Inputs (Level 2) | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		71,738	72,135
Significant Other Observable Inputs (Level 2) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,136,766	986,636
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		78,698	86,771
Liabilities measured at fair value		(63,930)	(60,996)
Significant Unobservable Inputs (Level 3) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		(63,930)	(60,996)
Significant Unobservable Inputs (Level 3) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	[1]	0	0
Significant Unobservable Inputs (Level 3) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	9,149	15,387
Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	8,773	9,460
Significant Unobservable Inputs (Level 3) | Fixed maturities | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	347	387
Significant Unobservable Inputs (Level 3) | Fixed maturities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	29	540
Significant Unobservable Inputs (Level 3) | Fixed maturities | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[2]	0	5,000
Significant Unobservable Inputs (Level 3) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			0
Significant Unobservable Inputs (Level 3) | Other investments at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			0
Significant Unobservable Inputs (Level 3) | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		69,549	71,384
Significant Unobservable Inputs (Level 3) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		11,335	12,217
Significant Unobservable Inputs (Level 3) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		0	0
Significant Unobservable Inputs (Level 3) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		58,214	59,167
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		20,133,567	20,802,739
Liabilities measured at fair value		(110,857)	(115,835)
Estimated Fair Value | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		(63,930)	(60,996)
Estimated Fair Value | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	[1]	(24,529)	(34,375)
Estimated Fair Value | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value		(22,398)	(20,464)
Estimated Fair Value | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		14,347,460	14,332,391
Estimated Fair Value | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		5,535,763	4,434,439
Estimated Fair Value | Fixed maturities | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		488,699	316,141
Estimated Fair Value | Fixed maturities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,431,256	2,158,840
Estimated Fair Value | Fixed maturities | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		590,198	545,817
Estimated Fair Value | Fixed maturities | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		2,788,272	3,484,257
Estimated Fair Value | Fixed maturities | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,692,783	1,612,754
Estimated Fair Value | Fixed maturities | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,820,489	1,780,143
Estimated Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,096,798	1,469,042
Estimated Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		546,075	504,333
Estimated Fair Value | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			264,989
Estimated Fair Value | Other investments at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value			76,427
Estimated Fair Value | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[3]		188,562
Estimated Fair Value | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		31,623	15,747
Estimated Fair Value | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		4,111,611	4,216,237
Estimated Fair Value | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		936,840	1,068,725
Estimated Fair Value | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		16,410	20,491
Estimated Fair Value | Fair value option | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		7,082	15,210
Estimated Fair Value | Fair value option | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,306	11,997
Estimated Fair Value | Fair value option | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		119,985	231,290
Estimated Fair Value | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		132,162	195,788
Estimated Fair Value | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		214,718	99,354
Estimated Fair Value | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		217,066	297,426
Estimated Fair Value | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		137,138	139,575
Estimated Fair Value | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value		1,260,645	1,128,094
Estimated Fair Value | Fair value option | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	[3]	$ 1,068,259	$ 1,008,287
[1]	Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets.
[2]	In securities lending transactions, the Company receives collateral in excess of the fair value of the securities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See Note 7, “Investment Information—Securities Lending Agreements.”
[3]	In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.

Fair Value - Rollforward of Lev

Fair Value - Rollforward of Level 3 assets and liabilities (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		$ 84,889	$ 54,238	$ 86,771	$ 54,633
Total gains or (losses) (realized/unrealized) - included in earnings		(198)	3,708	(987)	4,672
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		(320)	0	(176)	0
Purchases, issuances, sales and settlements
Purchases		393	4,935	393	4,935
Issuances		0	0	0	0
Sales		(5,077)	(26,242)	(5,077)	(26,242)
Settlements		(989)	(69)	(2,226)	(1,428)
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		78,698	36,570	78,698	36,570
Contingent consideration liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		(62,449)	(125,544)	(60,996)	(122,350)
Total gains or (losses) (realized/unrealized) - included in earnings		(1,481)	(3,441)	(2,934)	(7,087)
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		0	0	0	0
Settlements		0	71,739	0	72,191
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		(63,930)	(57,246)	(63,930)	(57,246)
Available for sale | Structured securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	[1]	5,413	10,637	5,927	11,289
Total gains or (losses) (realized/unrealized) - included in earnings	[1]	3	3,072	4	3,779
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	[1]	(4)	0	(8)	0
Purchases, issuances, sales and settlements
Purchases	[1]	0	0	0	0
Issuances	[1]	0	0	0	0
Sales	[1]	(5,003)	(13,640)	(5,003)	(13,640)
Settlements	[1]	(33)	(69)	(544)	(1,428)
Transfers in and/or out of Level 3	[1]	0	0	0	0
Balance at end of period	[1]	376	0	376	0
Available for sale | Corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		9,152	18,601	9,460	18,344
Total gains or (losses) (realized/unrealized) - included in earnings		0	636	0	893
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		(316)	0	(168)	0
Purchases, issuances, sales and settlements
Purchases		393	4,935	393	4,935
Issuances		0	0	0	0
Sales		0	(12,602)	0	(12,602)
Settlements		(456)	0	(912)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		8,773	11,570	8,773	11,570
Fair value option | Corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		11,872	0	12,217	0
Total gains or (losses) (realized/unrealized) - included in earnings		(537)	0	(612)	0
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		0	0	0	0
Settlements		0	0	(270)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		11,335	0	11,335	0
Fair value option | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		58,452	25,000	59,167	25,000
Total gains or (losses) (realized/unrealized) - included in earnings		336	0	(379)	0
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		(74)	0	(74)	0
Settlements		(500)	0	(500)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		$ 58,214	$ 25,000	$ 58,214	$ 25,000
[1]	Includes asset backed securities, mortgage backed securities and commercial mortgage backed securities.

Fair Value - Narrative (Details

Fair Value - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Total assets and liabilities measured at fair value	$ 20,240,000	$ 20,920,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	$ 276,800	$ 181,500
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	1.40%	0.90%
Senior notes	$ 1,733,211	$ 1,732,884
Estimated fair value of senior notes	$ 1,890,000	$ 2,040,000

Derivative Instruments (Details

Derivative Instruments (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements		$ 44,000			$ 44,000			$ 40,600
Derivative liabilities subject to master netting agreements		20,100			20,100			19,600
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	[1]	(2,146)		$ (4,770)	(6,109)		$ (13,951)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value		46,384			46,384			42,931
Liability derivatives - fair value		(22,398)			(22,398)			(20,464)
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(2,146)		(4,770)	(6,109)		(13,951)
Not Designated as Hedging Instrument [Member] | Futures contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,276,189			1,276,189			1,452,497
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(240)		(5,310)	4,790		2,410
Not Designated as Hedging Instrument [Member] | Futures contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value		556	[3]		556	[3]		3,371
Not Designated as Hedging Instrument [Member] | Futures contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(3,425)			(3,425)			(1,542)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,072,743			1,072,743			686,941
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(1,692)		(272)	(7,616)		(12,038)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	18,183			18,183			4,478
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(7,732)			(7,732)			(4,381)
Not Designated as Hedging Instrument [Member] | TBAs
Derivative [Line Items]
Net derivatives - notional value	[2],[4]	14,480			14,480			27,066
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		0		86	(97)		21
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale
Derivative [Line Items]
Asset derivatives - fair value		14,761	[4]		14,761	[4]		27,184
Liability derivatives - fair value	[4]	0			0			0
Not Designated as Hedging Instrument [Member] | Other
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,924,299			1,924,299			1,457,345
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(214)		$ 726	(3,186)		$ (4,344)
Not Designated as Hedging Instrument [Member] | Other | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	12,884			12,884			7,898
Not Designated as Hedging Instrument [Member] | Other | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	$ (11,241)			$ (11,241)			$ (14,541)
[1]	See Note 9 for information on the Company’s derivative instruments.
[2]	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.
[3]	The fair value of asset derivatives are included in ‘other assets’ and the fair value of liability derivatives are included in ‘other liabilities.’
[4]	The fair value of TBAs are included in ‘fixed maturities available for sale, at fair value.’

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Investment commitments	$ 1,630		$ 1,700
Interest expense on senior notes and other borrowings	$ 60.9	$ 58

Variable Interest Entities an63

Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	$ 4,111,611		$ 4,216,237
Fixed maturities available for sale, at fair value	14,128,989		13,876,003
Equity securities, at fair value	534,482		495,804
Cash	526,628		606,199
Accrued investment income	114,307		113,133
Premiums receivable	1,351,310		1,135,249
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,727,303		2,540,143
Ceded unearned premiums	1,014,663		926,611
Deferred acquisition costs net	569,817		535,824
Receivable for securities sold	143,809		205,536
Goodwill and intangible assets	593,008		652,611	$ 712,975
Other assets	1,000,471		1,053,009
Total assets	31,853,120		32,051,658
Reserve for losses and loss adjustment expenses	11,424,337	$ 11,496,205	11,383,792	10,520,511	$ 10,296,821	$ 10,200,960
Unearned premiums	3,833,540		3,622,314
Reinsurance balances payable	411,082		323,496
Revolving credit agreement borrowings	572,289		816,132
Payable for securities purchased	356,583		449,186
Other liabilities	752,399		782,717
Total liabilities of consolidated VIE	21,622,107		21,805,723
Redeemable noncontrolling interests	206,105		205,922
Watford Re
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests	206,105	$ 206,013	205,922	$ 205,736	$ 205,644	$ 205,553
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	2,385,316		2,426,066
Fixed maturities available for sale, at fair value	279,177		0
Equity securities, at fair value	63,010		0
Cash	45,644		54,503
Accrued investment income	18,108		18,261
Premiums receivable	232,842		177,492
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	54,299		42,777
Ceded unearned premiums	55,853		24,762
Deferred acquisition costs net	88,752		85,961
Receivable for securities sold	31,387		36,374
Goodwill and intangible assets	7,650		7,650
Other assets	75,478		140,808
Total assets	3,337,516		3,014,654
Reserve for losses and loss adjustment expenses	899,395		798,262
Unearned premiums	384,537		330,644
Reinsurance balances payable	41,185		18,424
Revolving credit agreement borrowings	447,289		441,132
Payable for securities purchased	132,164		42,501
Other liabilities	244,320		215,186
Total liabilities of consolidated VIE	2,148,890		1,846,149
Redeemable noncontrolling interests	$ 220,805		$ 220,622

Variable Interest Entities an64

Variable Interest Entities and Noncontrolling Interests - Variable interest entity (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2014	Jun. 30, 2018	Jun. 30, 2017
Variable Interest Entity [Line Items]
Net cash provided by operating activities		$ 496,886	$ 533,535
Net cash used for investing activities		102,399	(289,115)
Net cash used for financing activities		(733,029)	(327,790)
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Initial investment contribution amount	$ 100,000
Ownership percentage	11.00%
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Net cash provided by operating activities		92,400	134,400
Net cash used for investing activities		(168,000)	(63,500)
Net cash used for financing activities		$ (2,100)	$ (76,400)

Variable Interest Entities an65

Variable Interest Entities and Noncontrolling Interests - Non-redeemable noncontrolling interests (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interests, beginning of period			$ 843,411
Amounts attributable to noncontrolling interests	$ 8,116	$ 9,346	19,492	$ 25,670
Non-redeemable noncontrolling interests, end of period	861,153	877,456	861,153	877,456
Watford Re
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interests, beginning of period	854,112	868,186	843,411	851,854
Amounts attributable to noncontrolling interests	8,116	9,346	19,492	25,670
Other comprehensive income attributable to noncontrolling interests	(1,075)	(76)	(1,750)	(68)
Non-redeemable noncontrolling interests, end of period	$ 861,153	$ 877,456	$ 861,153	$ 877,456
Common shares | Watford Re
Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	89.00%		89.00%

Variable Interest Entities an66

Variable Interest Entities and Noncontrolling Interests - Redeemable noncontrolling interests (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interests, beginning of period			$ 205,922
Redeemable noncontrolling interests, end of period	$ 206,105		206,105
Watford Re
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interests, beginning of period	206,013	$ 205,644	205,922	$ 205,553
Accretion of preference share issuance costs	92	92	183	183
Redeemable noncontrolling interests, end of period	$ 206,105	$ 205,736	$ 206,105	$ 205,736
Cumulative redeemable preference shares | Watford Re
Redeemable Noncontrolling Interest [Line Items]
Preference shares, number of shares issued	9,065,200		9,065,200
Par value per share	$ 0.01		$ 0.01
Liquidation preference per share	$ 25		$ 25

Variable Interest Entities an67

Variable Interest Entities and Noncontrolling Interests - Income or loss attributable to third party investors (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Noncontrolling Interest [Abstract]
Amounts attributable to non-redeemable noncontrolling interests	$ (8,116)	$ (9,346)	$ (19,492)	$ (25,670)
Dividends attributable to redeemable noncontrolling interests	(4,585)	(4,586)	(9,170)	(9,170)
Net (income) loss attributable to noncontrolling interests	$ (12,701)	$ (13,932)	$ (28,662)	$ (34,840)

Variable Interest Entities an68

Variable Interest Entities and Noncontrolling Interests - Other variable interest entity disclosures (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 1,423,044	$ 1,171,792
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total VIE Assets	827,957	574,730
Maximum Exposure to Loss	6,722	4,108
Variable Interest Entity, Not Primary Beneficiary | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	2,334	882
Variable Interest Entity, Not Primary Beneficiary | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	4,388	3,226
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re I Ltd.
Variable Interest Entity [Line Items]
Total VIE Assets	69,079	92,390
Maximum Exposure to Loss	732	1,303
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re I Ltd. | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	263	471
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re I Ltd. | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	469	832
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re II Ltd.
Variable Interest Entity [Line Items]
Total VIE Assets	55,388	135,201
Maximum Exposure to Loss	124	547
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re II Ltd. | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	14	20
Variable Interest Entity, Not Primary Beneficiary | Bellemeade Re II Ltd. | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	110	527
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2017-1 Ltd.
Variable Interest Entity [Line Items]
Total VIE Assets	329,030	347,139
Maximum Exposure to Loss	2,574	2,258
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2017-1 Ltd. | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	815	391
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2017-1 Ltd. | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,759	$ 1,867
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-1 Ltd.
Variable Interest Entity [Line Items]
Total VIE Assets	374,460
Maximum Exposure to Loss	3,292
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-1 Ltd. | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,242
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-1 Ltd. | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 2,050

Other Comprehensive Income (L69

Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (470)	$ (1,730)	$ (632)	$ (3,537)
Income before income taxes	280,015	233,268	468,314	535,700
Income tax expense	(23,668)	(34,169)	(45,583)	(62,566)
Net of tax	243,646	185,167	394,069	438,294
Reclassification out of accumulated other comprehensive income | Unrealized appreciation on available-for-sale investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses)	(38,935)	24,111	(106,521)	31,924
Other-than-temporary impairment losses	(470)	(1,730)	(632)	(3,537)
Income before income taxes	(39,405)	22,381	(107,153)	28,387
Income tax expense	2,762	(5,157)	8,049	(6,119)
Net of tax	$ (36,643)	$ 17,224	$ (99,104)	$ 22,268

Other Comprehensive Income (L70

Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ (88,034)	$ 108,011	$ (277,977)	$ 219,483
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	0	0	0	0
Less reclassification of net realized gains included in net income, before tax	(39,405)	22,381	(107,153)	28,387
Foreign currency translation adjustments, before tax	(12,701)	18,509	(11,269)	21,674
Other comprehensive income (loss), before tax	(61,330)	104,139	(182,093)	212,770
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	(2,763)	15,042	(26,029)	25,722
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), tax	0	0	0	0
Less reclassification of net realized gains included in net income, tax	(2,762)	5,157	(8,049)	6,119
Foreign currency translation adjustments, tax	(106)	212	44	253
Other comprehensive income (loss), tax	(107)	10,097	(17,936)	19,856
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	(85,271)	92,969	(251,948)	193,761
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	0	0	0	0
Less reclassification of net realized gains included in net income, net of tax	(36,643)	17,224	(99,104)	22,268
Foreign currency translation adjustments, net of tax	(12,595)	18,297	(11,313)	21,421
Net current period other comprehensive income (loss)	$ (61,223)	$ 94,042	$ (164,157)	$ 192,914

Guarantor Financial Informati71

Guarantor Financial Information - Condensed consolidating balance sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Total investments	$ 21,537,418		$ 21,840,012
Cash	526,628		606,199
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	1,351,310		1,135,249
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,727,303		2,540,143
Contractholder receivables	2,044,322		1,978,414
Ceded unearned premiums	1,014,663		926,611
Deferred acquisition costs net	569,817		535,824
Goodwill and intangible assets	593,008		652,611	$ 712,975
Other assets	1,488,651		1,836,595
Total assets	31,853,120		32,051,658
Liabilities
Reserve for losses and loss adjustment expenses	11,424,337	$ 11,496,205	11,383,792	10,520,511	$ 10,296,821	$ 10,200,960
Unearned premiums	3,833,540		3,622,314
Reinsurance balances payable	411,082		323,496
Contractholder payables	2,044,322		1,978,414
Collateral held for insured obligations	257,396		240,183
Senior notes	1,733,211		1,732,884
Revolving credit agreement borrowings	572,289		816,132
Due to subsidiaries and affiliates	0		0
Other liabilities	1,345,930		1,708,508
Total liabilities	21,622,107		21,805,723
Redeemable noncontrolling interests	206,105		205,922
Shareholders' Equity
Total shareholders’ equity available to Arch	9,163,755		9,196,602	8,898,887
Non-redeemable noncontrolling interests	861,153		843,411	877,456
Total shareholders’ equity	10,024,908		10,040,013	$ 9,776,343
Total liabilities, noncontrolling interests and shareholders’ equity	31,853,120		32,051,658
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Assets
Total investments	134		96,540
Cash	7,033		9,997
Investments in subsidiaries	9,455,746		9,396,621
Due from subsidiaries and affiliates	0		394
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Ceded unearned premiums	0		0
Deferred acquisition costs net	0		0
Goodwill and intangible assets	0		0
Other assets	12,978		13,176
Total assets	9,475,891		9,516,728
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Collateral held for insured obligations	0		0
Senior notes	297,101		297,053
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	1,824		235
Other liabilities	13,211		22,838
Total liabilities	312,136		320,126
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	9,163,755		9,196,602
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	9,163,755		9,196,602
Total liabilities, noncontrolling interests and shareholders’ equity	9,475,891		9,516,728
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	198,122		46,281
Cash	27,513		30,380
Investments in subsidiaries	4,033,568		4,097,765
Due from subsidiaries and affiliates	2		0
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Ceded unearned premiums	0		0
Deferred acquisition costs net	0		0
Goodwill and intangible assets	0		0
Other assets	56,042		49,585
Total assets	4,315,247		4,224,011
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Collateral held for insured obligations	0		0
Senior notes	494,672		494,621
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	536,747		536,919
Other liabilities	29,304		29,317
Total liabilities	1,060,723		1,060,857
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	3,254,524		3,163,154
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	3,254,524		3,163,154
Total liabilities, noncontrolling interests and shareholders’ equity	$ 4,315,247		4,224,011
Percentage Ownership of Subsidiary	100.00%
Reportable Legal Entities | Other Arch Capital Subsidiaries
Assets
Total investments	$ 21,353,862		21,711,891
Cash	492,082		565,822
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	1,849,468		1,828,864
Premiums receivable	1,897,517		2,967,701
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	8,441,528		8,442,192
Contractholder receivables	2,044,322		1,978,414
Ceded unearned premiums	1,821,539		2,165,789
Deferred acquisition costs net	633,454		693,053
Goodwill and intangible assets	593,008		652,611
Other assets	1,555,017		1,860,505
Total assets	40,681,797		42,866,842
Liabilities
Reserve for losses and loss adjustment expenses	16,855,050		17,236,401
Unearned premiums	4,640,416		4,861,491
Reinsurance balances payable	957,289		2,155,947
Contractholder payables	2,044,322		1,978,414
Collateral held for insured obligations	257,396		240,183
Senior notes	941,438		941,210
Revolving credit agreement borrowings	572,289		816,132
Due to subsidiaries and affiliates	1,310,899		1,292,104
Other liabilities	1,785,950		1,949,696
Total liabilities	29,365,049		31,471,578
Redeemable noncontrolling interests	220,805		220,622
Shareholders' Equity
Total shareholders’ equity available to Arch	10,234,790		10,331,231
Non-redeemable noncontrolling interests	861,153		843,411
Total shareholders’ equity	11,095,943		11,174,642
Total liabilities, noncontrolling interests and shareholders’ equity	40,681,797		42,866,842
Consolidating Adjustments and Eliminations
Assets
Total investments	(14,700)		(14,700)
Cash	0		0
Investments in subsidiaries	(13,489,314)		(13,494,386)
Due from subsidiaries and affiliates	(1,849,470)		(1,829,258)
Premiums receivable	(546,207)		(1,832,452)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(5,714,225)		(5,902,049)
Contractholder receivables	0		0
Ceded unearned premiums	(806,876)		(1,239,178)
Deferred acquisition costs net	(63,637)		(157,229)
Goodwill and intangible assets	0		0
Other assets	(135,386)		(86,671)
Total assets	(22,619,815)		(24,555,923)
Liabilities
Reserve for losses and loss adjustment expenses	(5,430,713)		(5,852,609)
Unearned premiums	(806,876)		(1,239,177)
Reinsurance balances payable	(546,207)		(1,832,451)
Contractholder payables	0		0
Collateral held for insured obligations			0
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	(1,849,470)		(1,829,258)
Other liabilities	(482,535)		(293,343)
Total liabilities	(9,115,801)		(11,046,838)
Redeemable noncontrolling interests	(14,700)		(14,700)
Shareholders' Equity
Total shareholders’ equity available to Arch	(13,489,314)		(13,494,385)
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	(13,489,314)		(13,494,385)
Total liabilities, noncontrolling interests and shareholders’ equity	$ (22,619,815)		$ (24,555,923)

Guarantor Financial Informati72

Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net premiums earned	$ 1,336,763	$ 1,240,874	$ 2,571,662	$ 2,357,891
Net investment income	135,668	111,124	262,392	228,998
Net realized gains (losses)	(76,611)	21,735	(187,609)	55,888
Net impairment losses recognized in earnings	(470)	(1,730)	(632)	(3,537)
Other underwriting income	3,874	4,822	9,223	9,455
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541	80,794
Other income (loss)	3,113	(1,994)	3,187	(2,776)
Total revenues	1,410,809	1,407,537	2,694,764	2,726,713
Expenses
Losses and loss adjustment expenses	726,153	689,860	1,363,013	1,242,430
Acquisition expenses	202,838	190,436	394,214	372,725
Other operating expenses	176,181	169,981	351,196	344,700
Corporate expenses	22,512	24,876	37,824	52,668
Amortization of intangible assets	26,472	30,824	53,208	62,118
Interest expense	30,344	28,749	60,980	57,425
Net foreign exchange (gains) losses	(53,706)	39,543	(33,985)	58,947
Total expenses	1,130,794	1,174,269	2,226,450	2,191,013
Income (loss) before income taxes	280,015	233,268	468,314	535,700
Income tax (expense) benefit	(23,668)	(34,169)	(45,583)	(62,566)
Income (loss) before equity in net income of subsidiaries	256,347	199,099	422,731	473,134
Equity in net income of subsidiaries	0	0	0	0
Net income	256,347	199,099	422,731	473,134
Net (income) loss attributable to noncontrolling interests	(12,701)	(13,932)	(28,662)	(34,840)
Net income available to Arch	243,646	185,167	394,069	438,294
Preferred dividends	(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares	0	0	(2,710)	0
Net income available to Arch common shareholders	233,243	173,818	370,519	415,727
Comprehensive income available to Arch	183,500	279,285	231,662	631,276
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Revenues
Net premiums earned	0	0	0	0
Net investment income	15	1	35	6
Net realized gains (losses)	0	0	29	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	2,339	(437)	2,261	(266)
Total revenues	2,354	(436)	2,325	(260)
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	16,642	21,816	32,811	39,063
Amortization of intangible assets	0	0	0	0
Interest expense	5,537	6,075	11,073	12,090
Net foreign exchange (gains) losses	0	0	29	0
Total expenses	22,179	27,891	43,913	51,153
Income (loss) before income taxes	(19,825)	(28,327)	(41,588)	(51,413)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	(19,825)	(28,327)	(41,588)	(51,413)
Equity in net income of subsidiaries	263,471	213,494	435,657	489,707
Net income	243,646	185,167	394,069	438,294
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	243,646	185,167	394,069	438,294
Preferred dividends	(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares			(2,710)
Net income available to Arch common shareholders	233,243	173,818	370,519	415,727
Comprehensive income available to Arch	183,500	279,285	231,662	631,276
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned	0	0	0	0
Net investment income	560	184	818	1,000
Net realized gains (losses)	0	0	(7)	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	0	0	0	0
Total revenues	560	184	811	1,000
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	470	1,309	759	3,317
Amortization of intangible assets	0	0	0	0
Interest expense	12,013	11,989	23,939	23,919
Net foreign exchange (gains) losses	0	0	0	0
Total expenses	12,483	13,298	24,698	27,236
Income (loss) before income taxes	(11,923)	(13,114)	(23,887)	(26,236)
Income tax (expense) benefit	2,477	4,069	5,428	8,942
Income (loss) before equity in net income of subsidiaries	(9,446)	(9,045)	(18,459)	(17,294)
Equity in net income of subsidiaries	86,727	86,156	173,147	163,529
Net income	77,281	77,111	154,688	146,235
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	77,281	77,111	154,688	146,235
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	77,281	77,111	154,688	146,235
Comprehensive income available to Arch	70,066	105,302	76,603	193,083
Reportable Legal Entities | Other Arch Capital Subsidiaries
Revenues
Net premiums earned	1,336,763	1,240,874	2,571,662	2,357,891
Net investment income	157,532	133,627	306,299	271,608
Net realized gains (losses)	(76,611)	21,735	(187,631)	55,888
Net impairment losses recognized in earnings	(470)	(1,730)	(632)	(3,537)
Other underwriting income	3,874	4,822	9,223	9,455
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541	80,794
Other income (loss)	774	(1,557)	926	(2,510)
Total revenues	1,430,334	1,430,477	2,736,388	2,769,589
Expenses
Losses and loss adjustment expenses	726,153	689,860	1,363,013	1,242,430
Acquisition expenses	202,838	190,436	394,214	372,725
Other operating expenses	176,181	169,981	351,196	344,700
Corporate expenses	5,400	1,751	4,254	10,288
Amortization of intangible assets	26,472	30,824	53,208	62,118
Interest expense	34,911	33,050	70,083	64,386
Net foreign exchange (gains) losses	(43,357)	29,843	(26,921)	45,191
Total expenses	1,128,598	1,145,745	2,209,047	2,141,838
Income (loss) before income taxes	301,736	284,732	527,341	627,751
Income tax (expense) benefit	(26,145)	(38,238)	(51,011)	(71,508)
Income (loss) before equity in net income of subsidiaries	275,591	246,494	476,330	556,243
Equity in net income of subsidiaries	0	0	0	0
Net income	275,591	246,494	476,330	556,243
Net (income) loss attributable to noncontrolling interests	(13,023)	(14,254)	(29,307)	(35,485)
Net income available to Arch	262,568	232,240	447,023	520,758
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	262,568	232,240	447,023	520,758
Comprehensive income available to Arch	212,802	475,747	291,883	699,920
Consolidating Adjustments and Eliminations
Revenues
Net premiums earned	0	0	0	0
Net investment income	(22,439)	(22,688)	(44,760)	(43,616)
Net realized gains (losses)	0	0	0	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	0	0	0	0
Total revenues	(22,439)	(22,688)	(44,760)	(43,616)
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	0	0	0	0
Amortization of intangible assets	0	0	0	0
Interest expense	(22,117)	(22,365)	(44,115)	(42,970)
Net foreign exchange (gains) losses	(10,349)	9,700	(7,093)	13,756
Total expenses	(32,466)	(12,665)	(51,208)	(29,214)
Income (loss) before income taxes	10,027	(10,023)	6,448	(14,402)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	10,027	(10,023)	6,448	(14,402)
Equity in net income of subsidiaries	(350,198)	(299,650)	(608,804)	(653,236)
Net income	(340,171)	(309,673)	(602,356)	(667,638)
Net (income) loss attributable to noncontrolling interests	322	322	645	645
Net income available to Arch	(339,849)	(309,351)	(601,711)	(666,993)
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	(339,849)	(309,351)	(601,711)	(666,993)
Comprehensive income available to Arch	$ (282,868)	$ (581,049)	$ (368,486)	$ (893,003)

Guarantor Financial Informati73

Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net Cash Provided by (Used in) Operating Activities		$ 496,886	$ 533,535
Investing Activities
Purchases of fixed maturity investments		(16,867,570)	(19,899,326)
Purchases of equity securities		(679,663)	(400,155)
Purchases of other investments		(1,017,147)	(883,704)
Proceeds from sales of fixed maturity investments		16,090,543	19,611,680
Proceeds from sales of equity securities		622,068	473,064
Proceeds from sales, redemptions and maturities of other investments		773,298	614,494
Proceeds from redemptions and maturities of fixed maturity investments		511,448	447,941
Net settlements of derivative instruments		4,498	(5,984)
Net (purchases) sales of short-term investments		451,901	(445,203)
Change in cash collateral related to securities lending		176,304	175,693
Contributions to subsidiaries		0	0
Issuance of intercompany loans			0
Repayment of intercompany loans			0
Purchases of fixed assets		(13,242)	(11,103)
Other		49,961	33,488
Net Cash Provided By (Used For) Investing Activities		102,399	(289,115)
Financing Activities
Redemption of preferred shares		(92,555)	0
Purchases of common shares under share repurchase program		(173,575)	0
Proceeds from common shares issued, net		(13,851)	(6,838)
Proceeds from borrowings		130,579	0
Repayments of borrowings		(373,000)	(72,000)
Change in cash collateral related to securities lending		(176,304)	(175,693)
Dividends paid to redeemable noncontrolling interests		(8,994)	(8,994)
Dividends paid to parent	[1]	0	0
Other		(4,489)	(41,698)
Preferred dividends paid		(20,840)	(22,567)
Net Cash Provided By (Used For) Financing Activities		(733,029)	(327,790)
Effects of exchange rate changes on foreign currency cash and restricted cash		(10,431)	9,616
Increase (decrease) in cash and restricted cash		(144,175)	(73,754)
Cash and restricted cash, beginning of year		727,284	969,569
Cash and restricted cash, end of period		583,109	895,815
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Operating Activities
Net Cash Provided by (Used in) Operating Activities		201,487	7,483
Investing Activities
Purchases of fixed maturity investments		0	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		0	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	0
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		96,446	2,354
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		0	20,641
Issuance of intercompany loans			0
Repayment of intercompany loans			0
Purchases of fixed assets		(71)	(18)
Other		(4)	0
Net Cash Provided By (Used For) Investing Activities		96,371	22,977
Financing Activities
Redemption of preferred shares		(92,555)
Purchases of common shares under share repurchase program		(173,575)
Proceeds from common shares issued, net		(13,851)	(6,838)
Proceeds from borrowings		0
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		0	0
Dividends paid to parent	[1]	0	0
Other		0	0
Preferred dividends paid		(20,840)	(22,567)
Net Cash Provided By (Used For) Financing Activities		(300,821)	(29,405)
Effects of exchange rate changes on foreign currency cash and restricted cash		0	0
Increase (decrease) in cash and restricted cash		(2,963)	1,055
Cash and restricted cash, beginning of year		10,048	1,738
Cash and restricted cash, end of period		7,085	2,793
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Operating Activities
Net Cash Provided by (Used in) Operating Activities		149,576	27,853
Investing Activities
Purchases of fixed maturity investments		(125,440)	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		33,793	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	0
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		(59,798)	(27,896)
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		(1,000)	(72,900)
Issuance of intercompany loans			0
Repayment of intercompany loans			47,000
Purchases of fixed assets		0	(10)
Other		0	0
Net Cash Provided By (Used For) Investing Activities		(152,445)	(53,806)
Financing Activities
Redemption of preferred shares		0
Purchases of common shares under share repurchase program		0
Proceeds from common shares issued, net		0	0
Proceeds from borrowings		0
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		0	0
Dividends paid to parent	[1]	0	0
Other		0	0
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		0	0
Effects of exchange rate changes on foreign currency cash and restricted cash		0	0
Increase (decrease) in cash and restricted cash		(2,869)	(25,953)
Cash and restricted cash, beginning of year		30,380	71,955
Cash and restricted cash, end of period		27,511	46,002
Reportable Legal Entities | Other Arch Capital Subsidiaries
Operating Activities
Net Cash Provided by (Used in) Operating Activities		1,010,703	956,437
Investing Activities
Purchases of fixed maturity investments		(17,347,846)	(19,899,326)
Purchases of equity securities		(679,663)	(400,155)
Purchases of other investments		(1,017,147)	(883,704)
Proceeds from sales of fixed maturity investments		16,662,466	19,611,680
Proceeds from sales of equity securities		622,068	473,064
Proceeds from sales, redemptions and maturities of other investments		773,298	614,494
Proceeds from redemptions and maturities of fixed maturity investments		511,448	447,941
Net settlements of derivative instruments		4,498	(5,984)
Net (purchases) sales of short-term investments		415,253	(419,661)
Change in cash collateral related to securities lending		176,304	175,693
Contributions to subsidiaries		(22,595)	(342,950)
Issuance of intercompany loans			(47,000)
Repayment of intercompany loans			0
Purchases of fixed assets		(13,171)	(11,075)
Other		49,965	54,129
Net Cash Provided By (Used For) Investing Activities		134,878	(632,854)
Financing Activities
Redemption of preferred shares		0
Purchases of common shares under share repurchase program		0
Proceeds from common shares issued, net		23,595	395,209
Proceeds from borrowings		130,579
Repayments of borrowings		(373,000)	(72,000)
Change in cash collateral related to securities lending		(176,304)	(175,693)
Dividends paid to redeemable noncontrolling interests		(9,632)	(9,632)
Dividends paid to parent	[1]	(864,242)	(457,600)
Other		(4,489)	(62,339)
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		(1,273,493)	(382,055)
Effects of exchange rate changes on foreign currency cash and restricted cash		(10,431)	9,616
Increase (decrease) in cash and restricted cash		(138,343)	(48,856)
Cash and restricted cash, beginning of year		686,856	895,876
Cash and restricted cash, end of period		548,513	847,020
Consolidating Adjustments and Eliminations
Operating Activities
Net Cash Provided by (Used in) Operating Activities		(864,880)	(458,238)
Investing Activities
Purchases of fixed maturity investments		605,716	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		(605,716)	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	0
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		0	0
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		23,595	395,209
Issuance of intercompany loans			47,000
Repayment of intercompany loans			(47,000)
Purchases of fixed assets		0	0
Other		0	(20,641)
Net Cash Provided By (Used For) Investing Activities		23,595	374,568
Financing Activities
Redemption of preferred shares		0
Purchases of common shares under share repurchase program		0
Proceeds from common shares issued, net		(23,595)	(395,209)
Proceeds from borrowings		0
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		638	638
Dividends paid to parent	[1]	864,242	457,600
Other		0	20,641
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		841,285	83,670
Effects of exchange rate changes on foreign currency cash and restricted cash		0	0
Increase (decrease) in cash and restricted cash		0	0
Cash and restricted cash, beginning of year		0	0
Cash and restricted cash, end of period		$ 0	$ 0
[1]	Dividends received by parent are included in net cash provided by (used for) operating activities

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective tax rate on income before income taxes (percentage)	9.70%	11.70%
Statutory federal tax rate	21.00%
Tax benefit on share-based compensation	0.60%
Net deferred tax assets	$ 9.7		$ 39.6
Income taxes paid	$ (46.5)	$ 3.9