Senior Notes	12 Months Ended
Dec. 31, 2014
Debt Disclosure [Abstract]
Senior Notes	Senior Notes On May 4, 2004, ACGL completed a public offering of $300.0 million principal amount of 7.35% senior notes due May 1, 2034 (“ACGL Senior Notes”). The ACGL Senior Notes are ACGL’s senior unsecured obligations and rank equally with all of its existing and future senior unsecured indebtedness. Interest payments on the ACGL Senior Notes are due on May 1st and November 1st of each year. ACGL may redeem the ACGL Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the ACGL Senior Notes at December 31, 2014 and 2013 was $462.4 million and $381.0 million, respectively. On December 13, 2013, Arch-U.S., a wholly-owned subsidiary of ACGL, completed a public offering of $500.0 million principal amount of 5.144% senior notes due November 1, 2043 (“Arch-U.S. Senior Notes”), fully and unconditionally guaranteed by ACGL (the “Guarantee”). The Arch-U.S. Senior Notes are unsecured and unsubordinated obligations of Arch-U.S. and ACGL, respectively, and rank equally and ratably with the other unsecured and unsubordinated indebtedness of Arch-U.S. and ACGL, respectively. Interest payments on the Arch-U.S. Senior Notes are due on May 1st and November 1st of each year. Arch-U.S. may redeem the Arch-U.S. Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the Arch-U.S. Senior Notes at December 31, 2014 and 2013 was $609.9 million and $514.1 million, respectively.

Senior Notes

On May 4, 2004, ACGL completed a public offering of $300.0 million principal amount of 7.35% senior notes due May 1, 2034 (“ACGL Senior Notes”). The ACGL Senior Notes are ACGL’s senior unsecured obligations and rank equally with all of its existing and future senior unsecured indebtedness. Interest payments on the ACGL Senior Notes are due on May 1st and November 1st of each year. ACGL may redeem the ACGL Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the ACGL Senior Notes at December 31, 2014 and 2013 was $462.4 million and $381.0 million, respectively.

On December 13, 2013, Arch-U.S., a wholly-owned subsidiary of ACGL, completed a public offering of $500.0 million principal amount of 5.144% senior notes due November 1, 2043 (“Arch-U.S. Senior Notes”), fully and unconditionally guaranteed by ACGL (the “Guarantee”). The Arch-U.S. Senior Notes are unsecured and unsubordinated obligations of Arch-U.S. and ACGL, respectively, and rank equally and ratably with the other unsecured and unsubordinated indebtedness of Arch-U.S. and ACGL, respectively. Interest payments on the Arch-U.S. Senior Notes are due on May 1st and November 1st of each year. Arch-U.S. may redeem the Arch-U.S. Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the Arch-U.S. Senior Notes at December 31, 2014 and 2013 was $609.9 million and $514.1 million, respectively.