Statutory and regulatory requirements (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) subsidiaries director	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Statutory Accounting Practices [Line Items]
Actual dividends received from subsidiaries	$ 8,000	$ 320,000
Bermuda
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	4,203,912	4,244,211
Statutory net income (loss)	13,449	477,624	$ 329,260
Unrestricted net assets available for dividend payments or return of capital	$ 1,517,694	1,506,210
Number of wholly owned subsidiaries | subsidiaries	5
Required statutory capital and surplus	$ 677,050	1,447,138
Prior Year Minimum Solvency Margin		732,253
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	$ 7,000	30,000
United States
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	376,178	416,746
Statutory net income (loss)	(7,968)	7,795	32,255
Required statutory capital and surplus	102,138	78,342
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval	0	490
Statutory Accounting Practices, Dividends Paid with Approval of Regulatory Agency	$ 15,500	18,500
Maximum Permitted Dividends Percentage Statutory Capital	10.00%
Switzerland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 846,152	700,776
Statutory net income (loss)	39,570	30,231	$ 21,379
Required statutory capital and surplus	$ 315,000	$ 267,000
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
Insurance Segment | Operating Segments
Statutory Accounting Practices [Line Items]
Number of wholly owned subsidiaries | subsidiaries	2
Insurance Segment | Operating Segments | Western World Insurance Group, Inc
Statutory Accounting Practices [Line Items]
Number of wholly owned subsidiaries | subsidiaries	3