Statutory and regulatory requirements (Statutory capital and surplus and statutory net income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Bermuda
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	$ 734,045	$ 1,463,408
Actual statutory capital and surplus	3,712,369	3,469,849
Statutory net income (loss)	369,684	329,260	$ 511,652
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	78,342	80,047
Actual statutory capital and surplus	416,746	421,047
Statutory net income (loss)	7,795	32,255	105,210
Switzerland
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	267,000	259,000
Actual statutory capital and surplus	806,099	750,356
Statutory net income (loss)	$ 46,362	$ 21,379	$ 44,939