Debt and financing arrangements (Narrative) (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Senior Notes and Junior Debentures
Long-term Debt		$ 537,987	$ 537,668
Credit Facilities
Credit facility, commitment		690,000	700,000
Credit facility, issued and outstanding	[1]	338,668	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	[1]	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	[1]	105,575	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	[1]	5,666	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	[1]	11,805	10,543
AlphaCat Re secured letter of credit facility
Credit Facilities
Credit facility, commitment		20,000	30,000
Credit facility, issued and outstanding	[1]	20,000	30,000
$236,000 Flagstone bi-lateral facility
Credit Facilities
Credit facility, commitment		236,000	236,000
Credit facility, issued and outstanding	[1]	195,622	$ 193,764
2010 Senior Notes Due 2040
Senior Notes and Junior Debentures
Long-term Debt		$ 250,000
2010 Senior Notes Due 2040 | Minimum
Senior Notes and Junior Debentures
Debt Instrument Redemption Required Notice Period		30 days
2010 Senior Notes Due 2040 | Maximum
Senior Notes and Junior Debentures
Debt Instrument Redemption Required Notice Period		60 days
Junior subordinated deeferrable debentures
Senior Notes and Junior Debentures
Long-term Debt		$ 537,987
Debt Instrument Redemption Period Duration		5 years
At Issuance | 2006 Junior Subordinated Deferrable Debentures
Debt and financing arrangements [Line Items]
Interest Rate Term		5 years
At Issuance | 2007 Junior Subordinated Deferrable Debentures
Debt and financing arrangements [Line Items]
Interest Rate Term		5 years
Three month LIBOR | At Issuance | 2006 Junior Subordinated Deferrable Debentures
Debt and financing arrangements [Line Items]
Spread on variable rate (in percent)		3.55%
Three month LIBOR | At Issuance | 2007 Junior Subordinated Deferrable Debentures
Debt and financing arrangements [Line Items]
Spread on variable rate (in percent)		2.95%

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