Derivatives and Hedging Activities - Outstanding Interest Rate Swaptions and Derivative Instrument Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($)
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	$ 9,663,596
Additions	4,964,187
Settlement, Termination, Expiration or Exercise	(4,020,391)
Notional amount as of September 31, 2020	10,607,392
Interest Rate Swaps
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	8,050,000
Additions	5,050,000
Settlement, Termination, Expiration or Exercise	(4,000,000)
Notional amount as of September 31, 2020	9,100,000
Currency Forward Contracts
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	13,596
Additions	14,187
Settlement, Termination, Expiration or Exercise	(20,391)
Notional amount as of September 31, 2020	7,392
TBA Purchase Contracts
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	1,600,000
Additions	10,050,000
Settlement, Termination, Expiration or Exercise	(8,350,000)
Notional amount as of September 31, 2020	3,300,000
TBA Sale Contracts
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	0
Additions	(10,150,000)
Settlement, Termination, Expiration or Exercise	8,350,000
Notional amount as of September 31, 2020	1,800,000
Interest Rate Swap, Forward Start Dates | 1-Month LIBOR
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	1,300,000
Notional amount as of September 31, 2020	1,300,000
Interest Rate Swap, Fixed Rate
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	6,300,000
Notional amount as of September 31, 2020	6,800,000
Interest Rate Swap, Variable Rate | 1-Month LIBOR
Derivative Interest Rate Swaptions
Notional amount as of December 31, 2020	1,750,000
Notional amount as of September 31, 2020	$ 2,300,000