Debt and financing arrangements (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt and financing arrangements [Line Items]
Amount	$ 250,000	$ 250,000
Debentures payable	537,226	537,668
Credit facility, commitment	690,000	700,000
Letters of credit issued and outstanding	265,039	479,088
Debentures and senior notes payable	782,588	782,829
Investments pledged as collateral	$ 442,184	826,535
Five Year Credit Facilities [Member]
Debt and financing arrangements [Line Items]
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
Minimum level of consolidated net worth	$ 2,600,000
Five-year syndicated secured credit facility [Member]
Debt and financing arrangements [Line Items]
Letter of credit facility period	5 years
Line of credit facility initiation date	Dec. 09, 2015
Credit facility, commitment	$ 300,000	300,000
Letters of credit issued and outstanding	90,252	235,540
Aggregate commitments, maximum	400,000
Investments pledged as collateral	$ 157,597	370,909
Five-year unsecured credit facility [Member]
Debt and financing arrangements [Line Items]
Letter of credit facility period	5 years
Line of credit facility initiation date	Dec. 09, 2015
Credit facility, commitment	$ 85,000	85,000
Letters of credit issued and outstanding	0	0
Aggregate commitments, maximum	150,000
IPC bi-lateral facility
Debt and financing arrangements [Line Items]
Credit facility, commitment	25,000	25,000
Letters of credit issued and outstanding	5,842	9,241
$24,000 secured bi-lateral letter of credit facility
Debt and financing arrangements [Line Items]
Credit facility, commitment	24,000	24,000
Letters of credit issued and outstanding	4,553	10,543
Investments pledged as collateral	48,097	47,607
AlphaCat Re secured letter of credit facility
Debt and financing arrangements [Line Items]
Credit facility, commitment	20,000	30,000
Letters of credit issued and outstanding	20,000	30,000
Investments pledged as collateral	20,032	30,153
$236,000 Flagstone bi-lateral facility
Debt and financing arrangements [Line Items]
Credit facility, commitment	236,000	236,000
Letters of credit issued and outstanding	144,392	193,764
Investments pledged as collateral	216,458	$ 377,866
2010 Senior Notes due 2040 [Member]
Debt and financing arrangements [Line Items]
Amount	$ 250,000
Junior Subordinated Deferrable Debentures[Member]
Debt and financing arrangements [Line Items]
Debt Instrument Redemption Period Duration	5 years
Debentures payable	$ 537,226
Minimum
Debt and financing arrangements [Line Items]
Debt Instrument Redemption Required Notice Period	30 days
Maximum
Debt and financing arrangements [Line Items]
Debt Instrument Redemption Required Notice Period	60 days