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.

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.