Loss and loss adjustment expense reserves - Schedule of activity in the reserve for losses and loss adjustment expenses (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves for loss and loss adjustment expenses, beginning of period	$ 67,271	$ 0
Less: loss and loss adjustment expenses recoverable, beginning of period	0	0
Net reserves for loss and loss adjustment expenses, beginning of period	67,271	0
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Current year	107,279	80,306
Prior years'	(3,600)	0
Total incurred loss and loss adjustment expenses	103,679	80,306
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Current year	(14,951)	(13,035)
Prior years'	(18,576)	0
Total net paid losses	(33,527)	(13,035)
Net reserve for loss and loss adjustment expenses, end of period	137,423	67,271
Plus: loss and loss adjustment expenses recoverable, end of period	6,284	0
Gross reserve for loss and loss adjustment expenses, end of period	$ 143,707	$ 67,271