Debt and financing arrangements (Text) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Senior Notes and Junior Debentures
Long-term Debt	$ 541,454	$ 541,416
Credit Facilities
Credit facility, commitment	1,605,000	1,599,800
Credit facility, issued and outstanding	690,541	751,250
Credit Facilities
Credit Facilities
Minimum level of consolidated net worth	2,600,000
Percent of consolidated net income quarterly increase under covenant	50.00%
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
Four-year syndicated unsecured credit facility
Credit Facilities
Credit facility, commitment	400,000	400,000
Credit facility, issued and outstanding	0	0
Letter of credit facility period	4 years
Aggregate commitments, maximum	500,000
Line of credit facility initiation date	Mar. 09, 2012
Four-year syndicated secured credit facility
Credit Facilities
Credit facility, commitment	525,000	525,000
Credit facility, issued and outstanding	309,422	358,567
Letter of credit facility period	4 years
Aggregate commitments, maximum	700,000
Line of credit facility initiation date	Mar. 09, 2012
Bilateral unsecured letter of credit facility | Terminated Credit Facilities [Member]
Credit Facilities
Credit facility, commitment	60,000
Letter of credit facility period	3 years
Three year unsecured letter of credit facility | Terminated Credit Facilities [Member]
Credit Facilities
Credit facility, commitment	340,000
Letter of credit facility period	3 years
Five year secured letter of credit facility
Credit Facilities
Credit facility, commitment	525,000	525,000
Credit facility, issued and outstanding	309,422	358,567
Five year secured letter of credit facility | Terminated Credit Facilities [Member]
Credit Facilities
Credit facility, commitment	500,000
Letter of credit facility period	5 years
Talbot FAL Facility
Credit Facilities
Credit facility, commitment	25,000	25,000
Credit facility, issued and outstanding	25,000	25,000
IPC bi-lateral facility
Credit Facilities
Credit facility, commitment	40,000	40,000
Credit facility, issued and outstanding	19,817	20,177
$200,000 secured bi-lateral letter of credit facility
Credit Facilities
Credit facility, commitment	200,000	200,000
Credit facility, issued and outstanding	18,676	16,726
PaCRe Senior secured letter of credit facility
Credit Facilities
Credit facility, commitment	10,000	10,000
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	10,000
Credit facility, issued and outstanding	294	294
AlphaCat Re secured letter of credit facility
Credit Facilities
Credit facility, commitment	30,000	24,800
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	24,800
Credit facility, issued and outstanding	24,800	24,800
$375,000 Flagstone bi-lateral facility
Credit Facilities
Credit facility, commitment	375,000	375,000
Credit facility, issued and outstanding	292,532	305,686
2010 Senior Notes Due 2040
Senior Notes and Junior Debentures
Long-term Debt	250,000
2010 Senior Notes Due 2040 | Maximum
Senior Notes and Junior Debentures
Debt Instrument Redemption Required Notice Period	60 days
2010 Senior Notes Due 2040 | Minimum
Senior Notes and Junior Debentures
Debt Instrument Redemption Required Notice Period	30 days
Junior subordinated deeferrable debentures
Senior Notes and Junior Debentures
Long-term Debt	$ 541,454
Debt Instrument Redemption Period Duration	5 years