Debt and financing arrangements (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Senior Notes and Junior Debentures
Debt instrument, face amount	$ 250,000	$ 250,000
Debentures payable	538,168	537,668
Credit Facilities
Credit facility, commitment	690,000	700,000
Credit facility, issued and outstanding	308,501	479,088
$85,000 syndicated unsecured letter of credit facility
Credit Facilities
Credit facility, commitment	$ 85,000	85,000
Line of credit facility initiation date	Dec. 09, 2015
Letter of credit facility period	5 years
Aggregate commitments, maximum	$ 150,000
Credit facility, issued and outstanding	0	0
$300,000 syndicated secured letter of credit facility
Credit Facilities
Credit facility, commitment	$ 300,000	300,000
Line of credit facility initiation date	Dec. 09, 2015
Letter of credit facility period	5 years
Aggregate commitments, maximum	$ 400,000
Credit facility, issued and outstanding	107,208	235,540
Five year credit facilities
Credit Facilities
Minimum level of consolidated net worth	$ 2,600,000
Percent of consolidated net income quarterly increase under covenant	25.00%
Percent of any net proceeds from issuance of common shares	50.00%
Ratio consolidated total debt to net worth under covenant	0.35:1.00
$25,000 IPC bi-lateral facility
Credit Facilities
Credit facility, commitment	$ 25,000	25,000
Credit facility, issued and outstanding	5,553	9,241
$24,000 secured bi-lateral letter of credit facility
Credit Facilities
Credit facility, commitment	24,000	24,000
Credit facility, issued and outstanding	11,726	10,543
AlphaCat Re secured letter of credit facility
Credit Facilities
Credit facility, commitment	20,000	30,000
Credit facility, issued and outstanding	20,000	30,000
$236,000 Flagstone bi-lateral facility
Credit Facilities
Credit facility, commitment	236,000	236,000
Credit facility, issued and outstanding	164,014	$ 193,764
2010 Senior Notes Due 2040
Senior Notes and Junior Debentures
Debt instrument, face amount	$ 250,000
2010 Senior Notes Due 2040 | Minimum
Senior Notes and Junior Debentures
Required notice period for redemption	30 days
2010 Senior Notes Due 2040 | Maximum
Senior Notes and Junior Debentures
Required notice period for redemption	60 days
Junior subordinated deferrable debentures
Senior Notes and Junior Debentures
Redemption period duration	5 years
Debentures payable	$ 538,168