Organization and basis of presentation - Capital and repurchases of common stock (Details)		6 Months Ended
	Dec. 21, 2012 USD ($)	Jun. 30, 2016 USD ($) servicer	Dec. 31, 2015 shares
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Proceeds from contributed capital	$ 100,000,000
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Number of separate servicers | servicer		3
Previously reported | Error in reporting repurchased shares
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Common stock repurchased during period | shares			1,645,075
Restatement adjustment | Error in reporting repurchased shares
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Treasury stock eliminated, value		$ 25,000,000