Debt and financing arrangements (Credit facilities outstanding) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Commitment	$ 690,000	$ 700,000
Drawn and outstanding	308,501	479,088
$85,000 syndicated unsecured letter of credit facility
Line of Credit Facility [Line Items]
Commitment	85,000	85,000
Drawn and outstanding	0	0
$300,000 syndicated secured letter of credit facility
Line of Credit Facility [Line Items]
Commitment	300,000	300,000
Drawn and outstanding	107,208	235,540
$24,000 secured bi-lateral letter of credit facility
Line of Credit Facility [Line Items]
Commitment	24,000	24,000
Drawn and outstanding	11,726	10,543
$20,000 AlphaCat Re secured letter of credit facility
Line of Credit Facility [Line Items]
Commitment	20,000	30,000
Drawn and outstanding	20,000	30,000
$25,000 IPC bi-lateral facility
Line of Credit Facility [Line Items]
Commitment	25,000	25,000
Drawn and outstanding	5,553	9,241
$236,000 Flagstone bi-lateral facility
Line of Credit Facility [Line Items]
Commitment	236,000	236,000
Drawn and outstanding	$ 164,014	$ 193,764