Debt and financing arrangements (Schedule of finance structure) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Debt instrument, commitment	$ 851,654	$ 851,616
Debt instrument, outstanding	791,454	791,416
Debt instrument, drawn	788,679	788,614
Credit facility, commitment	1,605,000	1,599,800
Credit facility, issued and outstanding	690,541	751,250
Total debt and financing arrangements, commitment	2,456,654	2,451,416
Total debt and financing arrangements, outstanding	1,481,995	1,542,666
$400,000 syndicated unsecured letter of credit facility
Debt Instrument [Line Items]
Credit facility, commitment	400,000	400,000
Credit facility, issued and outstanding	0	0
$525,000 syndicated secured letter of credit facility
Debt Instrument [Line Items]
Credit facility, commitment	525,000	525,000
Credit facility, issued and outstanding	309,422	358,567
$200,000 secured bi-lateral letter of credit facility
Debt Instrument [Line Items]
Credit facility, commitment	200,000	200,000
Credit facility, issued and outstanding	18,676	16,726
Talbot FAL facility
Debt Instrument [Line Items]
Credit facility, commitment	25,000	25,000
Credit facility, issued and outstanding	25,000	25,000
PaCRe Senior secured letter of credit facility
Debt Instrument [Line Items]
Credit facility, commitment	10,000	10,000
Credit facility, issued and outstanding	294	294
AlphaCat Re secured letter of credit facility
Debt Instrument [Line Items]
Credit facility, commitment	30,000	24,800
Credit facility, issued and outstanding	24,800	24,800
IPC bi-lateral facility
Debt Instrument [Line Items]
Credit facility, commitment	40,000	40,000
Credit facility, issued and outstanding	19,817	20,177
$375,000 Flagstone bi-lateral facility
Debt Instrument [Line Items]
Credit facility, commitment	375,000	375,000
Credit facility, issued and outstanding	292,532	305,686
Total debentures payable
Debt Instrument [Line Items]
Debt instrument, commitment	601,654	601,616
Debt instrument, outstanding	541,454	541,416
Debt instrument, drawn	541,454	541,416
2006 Junior Subordinated Deferrable Debentures
Debt Instrument [Line Items]
Debt instrument, commitment	150,000	150,000
Debt instrument, outstanding	150,000	150,000
Debt instrument, drawn	150,000	150,000
2007 Junior Subordinated Deferrable Debentures
Debt Instrument [Line Items]
Debt instrument, commitment	200,000	200,000
Debt instrument, outstanding	139,800	139,800
Debt instrument, drawn	139,800	139,800
Flagstone 2006 Junior Subordinated Deferrable Debentures
Debt Instrument [Line Items]
Debt instrument, commitment	137,904	137,866
Debt instrument, outstanding	137,904	137,866
Debt instrument, drawn	137,904	137,866
Flagstone 2007 Junior Subordinated Deferrable Debentures
Debt Instrument [Line Items]
Debt instrument, commitment	113,750	113,750
Debt instrument, outstanding	113,750	113,750
Debt instrument, drawn	113,750	113,750
2010 Senior Notes Due 2040
Debt Instrument [Line Items]
Debt instrument, commitment	250,000	250,000
Debt instrument, outstanding	250,000	250,000
Debt instrument, drawn	$ 247,225	$ 247,198