Reserve for Losses and Loss Adjustment Expenses - Catastrophe losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year	$ 1,092,175	$ 598,940	$ 2,487,212	$ 1,848,299
Catastrophe losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year	347,800
Underwriting segments | Reinsurance | Catastrophe losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year	133,400
Underwriting segments | Insurance | Catastrophe losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year	214,500
Underwriting segments | Other | Catastrophe losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Current year	$ 19,800