Offsetting assets and liabilities - Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Effect of offsetting recognized liabilities, Derivative
Gross Amounts of Recognized Liabilities	$ 16,585	$ 5,250
Effect of offsetting recognized liabilities, Total
Gross Amounts of Recognized Liabilities, Total	1,723,954	1,194,642
Gross Amounts Offset in the Consolidated Balance Sheets, Total	8,432
Liabilities Presented in the Consolidated Balance Sheets, Total	1,715,522	1,194,642
Financial Instruments, Total	1,698,937	1,189,392
Cash Collateral Paid, Total	11,656	5,250
Credit default swaps
Effect of offsetting recognized liabilities, Derivative
Gross Amounts of Recognized Liabilities		110
Liabilities Presented in the Consolidated Balance Sheets, Derivative		110
Cash Collateral Paid, Derivative		110
Interest Rate Swap
Effect of offsetting recognized liabilities, Derivative
Gross Amounts of Recognized Liabilities	25,017	4,890
Gross Amounts Offset in the Consolidated Balance Sheets, Derivative	8,432
Liabilities Presented in the Consolidated Balance Sheets, Derivative	16,585	4,890
Cash Collateral Paid, Derivative	11,656	4,890
FX forwards
Effect of offsetting recognized liabilities, Derivative
Gross Amounts of Recognized Liabilities		250
Liabilities Presented in the Consolidated Balance Sheets, Derivative		250
Cash Collateral Paid, Derivative		250
Secured borrowings
Effect of offsetting recognized liabilities, Borrowings
Gross Amounts of Recognized Liabilities, Borrowings	1,698,937	1,189,392
Liabilities Presented in the Consolidated Balance Sheets, Borrowings	1,698,937	1,189,392
Financial Instruments, Borrowings	$ 1,698,937	$ 1,189,392