Debt and financing arrangements (Schedule of finance structure) (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Debt Instruments [Line Items]
Debt instrument, commitment		$ 848,187	$ 847,868
Debt instrument, issued and outstanding	[1]	787,987	787,668
Debt instrument, drawn		783,248	782,829
Credit facility, commitment		690,000	700,000
Credit facility, issued and outstanding	[1]	338,668	479,088
Total debt and financing arrangements, commitment		1,538,187	1,547,868
Total debt and financing arrangements, issued and outstanding	[1]	1,126,655	1,266,756
$85,000 syndicated unsecured letter of credit facility
Debt Instruments [Line Items]
Credit facility, commitment		85,000	85,000
Credit facility, issued and outstanding	[1]	0	0
$300,000 syndicated secured letter of credit facility
Debt Instruments [Line Items]
Credit facility, commitment		300,000	300,000
Credit facility, issued and outstanding	[1]	105,575	235,540
$24,000 secured bi-lateral letter of credit facility
Debt Instruments [Line Items]
Credit facility, commitment		24,000	24,000
Credit facility, issued and outstanding	[1]	11,805	10,543
AlphaCat Re secured letter of credit facility
Debt Instruments [Line Items]
Credit facility, commitment		20,000	30,000
Credit facility, issued and outstanding	[1]	20,000	30,000
$25,000 IPC bi-lateral facility
Debt Instruments [Line Items]
Credit facility, commitment		25,000	25,000
Credit facility, issued and outstanding	[1]	5,666	9,241
$236,000 Flagstone bi-lateral facility
Debt Instruments [Line Items]
Credit facility, commitment		236,000	236,000
Credit facility, issued and outstanding	[1]	195,622	193,764
Total debentures payable
Debt Instruments [Line Items]
Debt instrument, commitment		598,187	597,868
Debt instrument, issued and outstanding	[1]	537,987	537,668
Debt instrument, drawn		537,987	537,668
2006 Junior Subordinated Deferrable Debentures
Debt Instruments [Line Items]
Debt instrument, commitment		150,000	150,000
Debt instrument, issued and outstanding	[1]	150,000	150,000
Debt instrument, drawn		150,000	150,000
2007 Junior Subordinated Deferrable Debentures
Debt Instruments [Line Items]
Debt instrument, commitment		200,000	200,000
Debt instrument, issued and outstanding	[1]	139,800	139,800
Debt instrument, drawn		139,800	139,800
Flagstone 2006 Junior Subordinated Deferrable Debentures
Debt Instruments [Line Items]
Debt instrument, commitment		134,437	134,118
Debt instrument, issued and outstanding	[1]	134,437	134,118
Debt instrument, drawn		134,437	134,118
Flagstone 2007 Junior Subordinated Deferrable Debentures
Debt Instruments [Line Items]
Debt instrument, commitment		113,750	113,750
Debt instrument, issued and outstanding	[1]	113,750	113,750
Debt instrument, drawn		113,750	113,750
2010 Senior Notes Due 2040
Debt Instruments [Line Items]
Debt instrument, commitment		250,000	250,000
Debt instrument, issued and outstanding	[1]	250,000	250,000
Debt instrument, drawn		$ 245,261	$ 245,161

[1] Indicates utilization of commitment amount, not necessarily drawn borrowings.