Investments (Investments pledged as collateral) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	$ 1,605,000	$ 1,599,800
Letters of credit issued and outstanding	690,541	751,250
Investments pledged as collateral	1,146,952	1,246,406
$400,000 syndicated unsecured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	400,000	400,000
Letters of credit issued and outstanding	0	0
Investments pledged as collateral	0	0
$525,000 syndicated secured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	525,000	525,000
Letters of credit issued and outstanding	309,422	358,567
Investments pledged as collateral	499,905	507,620
$200,000 secured bi-lateral letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	200,000	200,000
Letters of credit issued and outstanding	18,676	16,726
Investments pledged as collateral	35,225	130,256
Talbot FAL Facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	25,000	25,000
Letters of credit issued and outstanding	25,000	25,000
Investments pledged as collateral	30,897	30,801
PaCRe Senior secured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	10,000	10,000
Letters of credit issued and outstanding	294	294
Investments pledged as collateral	0	0
AlphaCat Re secured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	30,000	24,800
Letters of credit issued and outstanding	24,800	24,800
Investments pledged as collateral	30,022	24,806
IPC bi-lateral facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	40,000	40,000
Letters of credit issued and outstanding	19,817	20,177
Investments pledged as collateral	96,960	98,465
$375,000 Flagstone bi-lateral facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment	375,000	375,000
Letters of credit issued and outstanding	292,532	305,686
Investments pledged as collateral	$ 453,943	$ 454,458