Fair value measurements (Level 3 rollforward) (Details) - Recurring - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3, beginning of year	$ 245,837	$ 50,248
Purchases	156,681	230,070
Sales	(2,389)	(5,025)
Settlements	(69,621)	(27,429)
Net realized losses		(11)
Change in net unrealized (losses) gains	462	(2,016)
Level 3, end of year	330,970	245,837
Bank Loan Portfolio
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3, beginning of year	232,337	32,748
Purchases	87,345	230,070
Sales	(2,389)	(1,036)
Settlements	(69,496)	(27,429)
Net realized losses		0
Change in net unrealized (losses) gains	(1,301)	(2,016)
Level 3, end of year	246,496	232,337
Catastrophe bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3, beginning of year	13,500	17,500
Purchases	33,272	0
Sales	0	(3,989)
Settlements	(125)	0
Net realized losses		(11)
Change in net unrealized (losses) gains	1,728	0
Level 3, end of year	48,375	13,500
Fixed income investment funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3, beginning of year	0
Purchases	12,168
Sales	0
Settlements	0
Change in net unrealized (losses) gains	0
Level 3, end of year	12,168	0
Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3, beginning of year	0
Purchases	23,896
Sales	0
Settlements	0
Change in net unrealized (losses) gains	35
Level 3, end of year	$ 23,931	$ 0