Debt and financing arrangements (Credit and other facilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Credit facility, commitment	$ 690,000	$ 700,000
Letters of credit issued and outstanding	265,039	479,088
Five-year unsecured credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility, commitment	85,000	85,000
Letters of credit issued and outstanding	0	0
Five-year syndicated secured credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility, commitment	300,000	300,000
Letters of credit issued and outstanding	90,252	235,540
$24,000 secured bi-lateral letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment	24,000	24,000
Letters of credit issued and outstanding	4,553	10,543
AlphaCat Re secured letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment	20,000	30,000
Letters of credit issued and outstanding	20,000	30,000
IPC bi-lateral facility
Line of Credit Facility [Line Items]
Credit facility, commitment	25,000	25,000
Letters of credit issued and outstanding	5,842	9,241
$236,000 Flagstone bi-lateral facility
Line of Credit Facility [Line Items]
Credit facility, commitment	236,000	236,000
Letters of credit issued and outstanding	$ 144,392	$ 193,764