Commitments and Contingencies	6 Months Ended
Jun. 30, 2013
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Letter of Credit and Revolving Credit Facilities As of June 30, 2013, the Company had a $300 million unsecured revolving loan and letter of credit facility and a $500 million secured letter of credit facility (the “Credit Agreement”). The Credit Agreement expires on August 18, 2014. In addition, the Company had access to secured letter of credit facilities of approximately $113.9 million as of June 30, 2013, which are available on a limited basis and for limited purposes (together with the secured portion of the Credit Agreement and these letter of credit facilities, the “LOC Facilities”). At June 30, 2013, the Company had $412.7 million in outstanding letters of credit under the LOC Facilities, which were secured by investments with a fair value of $481.3 million, and had $100.0 million of borrowings outstanding under the Credit Agreement. The Company was in compliance with all covenants contained in the LOC Facilities at June 30, 2013. 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 $751.0 million at June 30, 2013. Acquisition of CMG Mortgage Insurance Company and Mortgage Insurance Operating Platform of PMI In February 2013, certain of the Company's U.S.-based subsidiaries (collectively “Arch U.S. MI”) entered into a definitive agreement to acquire (1) CMG Mortgage Insurance Company (“CMG MI”) from its current owners, PMI Mortgage Insurance Co. in rehabilitation (“PMI”), which has been under the receivership of the Arizona Department of Insurance since 2011, and CMFG Life Insurance Company, and (2) PMI's mortgage insurance operating platform and certain related assets from PMI. In connection with the closing of the transactions, PMI and an affiliate of the Company's U.S.-based subsidiaries will enter into a quota share reinsurance agreement pursuant to which such affiliate, as the reinsurer, will agree to provide 100% quota share indemnity reinsurance to PMI for all certificates of insurance that were issued by PMI between and including January 1, 2009 and December 31, 2011 that are not in default as of an agreed upon effective date. At closing, it is currently estimated that the Company's U.S.-based subsidiaries will pay aggregate consideration of approximately $300 million under all transaction documents. Additional amounts may be paid based on the actual results of CMG MI's pre-closing portfolio over an agreed upon period. In addition, the Company will enter into a services agreement with PMI to provide for necessary services to administer the run-off of PMI's legacy business at the direction of PMI. On June 20, 2013, the Arizona receivership court provided the required approval of the acquisition. The transaction is also subject to approvals of the applicable regulators and approvals by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation of Arch U.S. MI as an eligible insurance carrier in the U.S. mortgage insurance marketplace, as well as the satisfaction of customary closing conditions. In connection with obtaining such consents of regulatory authorities and government-sponsored entities, it is anticipated that Arch U.S. MI or its affiliates will be required to make certain financial commitments to CMG MI, the form and amount of which will be determined based upon discussions with such authorities and entities. Arch U.S. MI's obligation to the sellers to accept financial requirements imposed by regulatory authorities and government-sponsored entities will be determined on the basis of, among other things, the appropriateness of such requirements in light of Arch U.S. MI's business plan and the consistency of such requirements with those imposed on other active participants in the U.S. mortgage insurance industry, as described in the purchase agreements. If these approvals are obtained, it is expected the transaction will close during the latter part of 2013.

Commitments and Contingencies

Letter of Credit and Revolving Credit Facilities

As of June 30, 2013, the Company had a $300 million unsecured revolving loan and letter of credit facility and a $500 million secured letter of credit facility (the “Credit Agreement”). The Credit Agreement expires on August 18, 2014. In addition, the Company had access to secured letter of credit facilities of approximately $113.9 million as of June 30, 2013, which are available on a limited basis and for limited purposes (together with the secured portion of the Credit Agreement and these letter of credit facilities, the “LOC Facilities”). At June 30, 2013, the Company had $412.7 million in outstanding letters of credit under the LOC Facilities, which were secured by investments with a fair value of $481.3 million, and had $100.0 million of borrowings outstanding under the Credit Agreement. The Company was in compliance with all covenants contained in the LOC Facilities at June 30, 2013.

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 $751.0 million at June 30, 2013.

Acquisition of CMG Mortgage Insurance Company and Mortgage Insurance Operating Platform of PMI

In February 2013, certain of the Company's U.S.-based subsidiaries (collectively “Arch U.S. MI”) entered into a definitive agreement to acquire (1) CMG Mortgage Insurance Company (“CMG MI”) from its current owners, PMI Mortgage Insurance Co. in rehabilitation (“PMI”), which has been under the receivership of the Arizona Department of Insurance since 2011, and CMFG Life Insurance Company, and (2) PMI's mortgage insurance operating platform and certain related assets from PMI. In connection with the closing of the transactions, PMI and an affiliate of the Company's U.S.-based subsidiaries will enter into a quota share reinsurance agreement pursuant to which such affiliate, as the reinsurer, will agree to provide 100% quota share indemnity reinsurance to PMI for all certificates of insurance that were issued by PMI between and including January 1, 2009 and December 31, 2011 that are not in default as of an agreed upon effective date. At closing, it is currently estimated that the Company's U.S.-based subsidiaries will pay aggregate consideration of approximately $300 million under all transaction documents. Additional amounts may be paid based on the actual results of CMG MI's pre-closing portfolio over an agreed upon period. In addition, the Company will enter into a services agreement with PMI to provide for necessary services to administer the run-off of PMI's legacy business at the direction of PMI.

On June 20, 2013, the Arizona receivership court provided the required approval of the acquisition. The transaction is also subject to approvals of the applicable regulators and approvals by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation of Arch U.S. MI as an eligible insurance carrier in the U.S. mortgage insurance marketplace, as well as the satisfaction of customary closing conditions. In connection with obtaining such consents of regulatory authorities and government-sponsored entities, it is anticipated that Arch U.S. MI or its affiliates will be required to make certain financial commitments to CMG MI, the form and amount of which will be determined based upon discussions with such authorities and entities. Arch U.S. MI's obligation to the sellers to accept financial requirements imposed by regulatory authorities and government-sponsored entities will be determined on the basis of, among other things, the appropriateness of such requirements in light of Arch U.S. MI's business plan and the consistency of such requirements with those imposed on other active participants in the U.S. mortgage insurance industry, as described in the purchase agreements. If these approvals are obtained, it is expected the transaction will close during the latter part of 2013.