Debt and financing arrangements (Investments pledged as collateral) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investments pledged as collateral	$ 442,184	$ 826,535
Five-year syndicated secured credit facility [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investments pledged as collateral	157,597	370,909
$24,000 secured bi-lateral letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investments pledged as collateral	48,097	47,607
AlphaCat Re secured letter of credit facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investments pledged as collateral	20,032	30,153
$236,000 Flagstone bi-lateral facility
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investments pledged as collateral	$ 216,458	$ 377,866