Balance Sheet Offsetting - Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Total
Gross Amounts of Recognized Liabilities	$ 1,357,191	$ 2,175,157
Gross Amounts Offset in the Balance Sheet	(211)	(667)
Net Amounts of Liabilities Presented in the Balance Sheet	1,356,980	2,174,490
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,303,036)	(2,124,554)
Net Amount	53,944	49,936
Institutional Counterparties
Securities loaned:
Gross Amounts of Recognized Liabilities	982,070	1,555,964
Net Amounts of Liabilities Presented in the Balance Sheets	982,070	1,555,964
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(948,012)	(1,509,933)
Net Amount	34,058	46,031
Institutional Counterparties | Interest rate swaps
Derivatives:
Gross Amounts of Recognized Liabilities	2,147	178
Net Amounts of Liabilities Presented in the Balance Sheet	2,147	178
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,946)	(112)
Net Amount	201	66
Institutional Counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	347,344	586,651
Net Amounts of Liabilities Presented in the Balance Sheet	347,344	586,651
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(347,344)	(586,651)
Net Amount	0	0
Institutional Counterparties | Forward MBS Derivatives
Derivatives:
Gross Amounts of Recognized Liabilities	20,824	6,890
Gross Amounts Offset in the Balance Sheet	(211)	(667)
Net Amounts of Liabilities Presented in the Balance Sheet	20,613	6,223
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(928)	(2,384)
Net Amount	19,685	3,839
Customer Counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	4,806	25,474
Net Amounts of Liabilities Presented in the Balance Sheet	4,806	25,474
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(4,806)	(25,474)
Net Amount	$ 0	$ 0