Investments (Pledged investments) (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		$ 700,000	$ 1,595,000
Letters of credit issued and outstanding	[1]	479,088	561,390
Investments pledged as collateral		826,535	1,022,740
Syndicated unsecured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		85,000	400,000
Letters of credit issued and outstanding		0	0
Investments pledged as collateral		0	0
Four-year syndicated secured credit facility [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		300,000	525,000
Letters of credit issued and outstanding		235,540	276,455	[1]
Investments pledged as collateral		370,909	395,750
Secured bi-lateral letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		24,000	200,000
Letters of credit issued and outstanding	[1]	10,543	15,649
Investments pledged as collateral		47,607	35,645
Talbot FAL Facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment			25,000
Letters of credit issued and outstanding	[1]		25,000
Investments pledged as collateral			31,048
AlphaCat Re secured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		30,000	30,000
Letters of credit issued and outstanding	[1]	30,000	30,000
Investments pledged as collateral		30,153	30,078
IPC bi-lateral facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		25,000	40,000
Letters of credit issued and outstanding	[1]	9,241	15,897
Investments pledged as collateral		0	99,437
Flagstone bi-lateral facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Credit facility, commitment		236,000	375,000
Letters of credit issued and outstanding	[1]	193,764	198,389
Investments pledged as collateral		$ 377,866	$ 430,782

[1] Indicates utilization of commitment amount, not necessarily drawn borrowings