Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands, AUD in Millions	Sep. 30, 2015 USD ($) Credit_Facilities		Sep. 30, 2015 AUD Credit_Facilities		Dec. 31, 2014 USD ($) Credit_Facilities
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities	18		18		8
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity	$ 1,039,100	[1],[2],[3]			$ 968,900	[1],[2],[3]
Available letter of credit facilities - commitments	1,500,000				1,600,000
Available letter of credit facilities - in use	455,900				606,100
November 2013 credit agreement
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity	976,100				968,900
Available letter of credit facilities - commitments	1,000,000
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	4,850,000	[2],[4]			3,575,000	[2],[4]
Available letter of credit facilities - in use	$ 2,538,499	[2]			$ 1,790,561	[5]
Collateralized by certain assets of the Company’s investment portfolio	49.90%	[2]	49.90%	[2]	66.20%	[2]
2015 Goldman Sachs Facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 200,000
2013 Citi agreements | Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	345,000				$ 575,000
2013 Citi agreements | Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	345,000
Syndicated credit agreements
Line of Credit Facility [Line Items]
Optional increased capacity	500,000
Catlin Group Limited
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	1,000
Catlin Group Limited | Unsecured multi-bank facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	450,000
Catlin Group Limited | Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	10,000
Catlin Group Limited | Bilateral facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity	63,000
Available letter of credit facilities - commitments	$ 150,000		AUD 50
Number of letter of credit facilities available | Credit_Facilities	2		2
Catlin Bermuda | Bilateral facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	$ 136,000
Catlin Bermuda | LLoyd's syndicate | Bilateral facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 375,000
Number of letter of credit facilities available | Credit_Facilities	4		4
Catlin Re Switzerland | Bilateral facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	$ 49,000

[1] As of September 30, 2015 and December 31, 2014, $976.1 million and $968.9 million, respectively, of revolving credit was available under the Company's syndicated unsecured credit agreement, which currently provides for up to $1 billion of revolving credit loans. The Company has the option to increase the size of the facilities under the syndicated secured and unsecured credit agreements by an additional $500 million across both facilities. Two bilateral facilities assumed as a result of the Catlin Acquisition with commitments totaling $150.0 million also provide for revolving credit loans. As of September 30, 2015, $63.0 million of revolving credit was available to draw under these facilities.

[2] As of September 30, 2015 and December 31, 2014, there were eighteen and eight available letter of credit facilities, respectively.

[3] The credit agreements with Citicorp USA, Inc. currently provide for the issuance of letters of credit and revolving credit loans up to an aggregate amount of $345.0 million, compared to $575.0 million available under the credit agreements with Citicorp USA, Inc. as of December 31, 2014. As of September 30, 2015, $345.0 million of letters of credit were issued under these agreements, and therefore no amounts are included in this line as available to draw under these facilities. The Company also has the option to increase the maximum amount of the letters of credit and revolving credit loans available under its credit agreements with Citicorp USA, Inc. with the lender's and issuing lender's consent.

[4] The available letter of credit facilities include approximately $1 billion that is also included in the "revolving credit facilities" line in this table.

[5] "In Use/Outstanding" data represent September 30, 2015 and December 31, 2014 accreted values.