Financing arrangements - Summary of total borrowings under repurchase agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Agency RMBS [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	$ 1,805,054	$ 2,005,133
Non-Agency RMBS [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	499,851	784,897
ABS [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	10,548	22,761
CMBS [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	196,658	159,490
Residential Mortgage Loans [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	83,762	0
Gross Amounts of Recognized Liabilities [Member]
Schedule Of Total Borrowings Under Repurchase Agreements [Line Items]
Financing arrangements, net	$ 2,595,873	$ 2,972,281