Earnings per share (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Warrant or Right [Line Items]
Outstanding warrants	1,007,500	1,007,500	1,007,500
Unvested restricted stock units previously granted to the Manager	28,192,541	27,700,154
Restricted Stock Units (RSUs) [Member] | Manager [Member]
Class of Warrant or Right [Line Items]
Unvested restricted stock units previously granted to the Manager	60,000	20,003	40,006