Statutory and regulatory requirements (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) subsidiaries director	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Statutory Accounting Practices [Line Items]
Actual dividends received from subsidiaries	$ 320,000	$ 465,000
Bermuda
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	3,712,369	3,469,849
Statutory net income (loss)	369,684	329,260	$ 511,652
Unrestricted net assets available for dividend payments or return of capital	$ 1,335,300	1,008,367
Number of wholly owned subsidiaries | subsidiaries	5
Percentage Of Total Statutory Capital And Surplus Limit	25.00%
Payment Of Dividends Notification Period Required	7 days
Number Of Directors Required For Affidavit | director	2
Percentage Of Total Statutory Capital Limit	15.00%
United Kingdom
Statutory Accounting Practices [Line Items]
Actual dividends received from subsidiaries	$ 30,000	65,000
United States
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	416,746	421,047
Statutory net income (loss)	$ 7,795	32,255	105,210
Number of wholly owned subsidiaries | subsidiaries	3
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval	$ 490	17,599
Statutory Accounting Practices, Dividends Paid with Approval of Regulatory Agency	$ 18,500	80,300
Maximum Permitted Dividends Percentage Statutory Capital	10.00%
Switzerland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 806,099	750,356
Statutory net income (loss)	$ 46,362	$ 21,379	$ 44,939
Maximum Permitted Dividends Percentage Annual Profit	20.00%
Maximum Permitted Dividends Percentage Statutory Capital	50.00%
Special Purpose Insurer | Bermuda
Statutory Accounting Practices [Line Items]
Number of wholly owned subsidiaries | subsidiaries	2