Reserve for losses and loss expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Case reserves			$ 1,295,385	$ 1,278,697
IBNR			$ 1,740,602	1,717,870
Western World
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Fair value adjustment to net reserves					$ 15,586
Amortized to income during the period	$ 2,524	$ 8,639
Prior year development percentage	3.90%	4.40%
Remaining amount of fair value adjustments to loss reserves to be amortized during the period				$ 2,340