Reserve for losses and loss expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Reported Claims, Amount			$ 1,267,625	$ 1,278,697
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount			$ 1,855,092	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	$ 2,892	$ 6,115
Prior Year Development Percentage	4.40%	4.60%
Remaining amount of fair value adjustments to loss reserves to be amortized during the period				$ 4,864