Balance Sheet Offsetting - Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Total
Gross Amounts of Recognized Liabilities	$ 2,002,833	$ 2,175,157
Gross Amounts Offset in the Balance Sheet	(2,878)	(667)
Net Amounts of Liabilities Presented in the Balance Sheet	1,999,955	2,174,490
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,848,947)	(2,124,554)
Net Amount	151,008	49,936
Institutional Counterparties
Securities loaned:
Gross Amounts of Recognized Liabilities	1,167,559	1,555,964
Net Amounts of Liabilities Presented in the Balance Sheets	1,167,559	1,555,964
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,075,906)	(1,509,933)
Net Amount	91,653	46,031
Institutional Counterparties | Interest rate swaps
Derivatives:
Gross Amounts of Recognized Liabilities	201	178
Net Amounts of Liabilities Presented in the Balance Sheet	201	178
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments		(112)
Net Amount	201	66
Institutional Counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	690,178	586,651
Net Amounts of Liabilities Presented in the Balance Sheet	690,178	586,651
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(690,178)	(586,651)
Net Amount	0	0
Institutional Counterparties | Forward MBS Derivatives
Derivatives:
Gross Amounts of Recognized Liabilities	136,640	6,890
Gross Amounts Offset in the Balance Sheet	(2,878)	(667)
Net Amounts of Liabilities Presented in the Balance Sheet	133,762	6,223
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(74,608)	(2,384)
Net Amount	59,154	3,839
Customer Counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	8,255	25,474
Net Amounts of Liabilities Presented in the Balance Sheet	8,255	25,474
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(8,255)	(25,474)
Net Amount	$ 0	$ 0