Derivative Financial Instruments - Aggregate Fair Value and Notional Amount of Derivative Financial Instruments (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Fair Value	$ 13,097,000	$ (127,723,000)
Notional Amount	2,575,858,000	13,589,356,000
TBAs
Derivative [Line Items]
Notional Amount	245,000,000	6,610,000,000
Interest rate futures
Derivative [Line Items]
Notional Amount	40,000,000	226,000,000
Derivative Liabilities
Derivative [Line Items]
Fair Value	(1,612,000)	(163,424,000)
Notional Amount	361,651,000	6,929,494,000
Derivative Liabilities | Interest rate swaps
Derivative [Line Items]
Fair Value	(15,000)	(97,235,000)
Notional Amount	25,000,000	2,314,300,000
Derivative Liabilities | Interest rate swaps | Cash Flow Hedging
Derivative [Line Items]
Fair Value	0	(51,530,000)
Notional Amount	0	139,500,000
Derivative Liabilities | TBAs
Derivative [Line Items]
Fair Value	(263,000)	(13,359,000)
Notional Amount	105,000,000	4,160,000,000
Derivative Liabilities | Interest rate futures
Derivative [Line Items]
Fair Value	0	(10,000)
Notional Amount	0	12,300,000
Derivative Liabilities | Loan purchase and interest rate lock commitments
Derivative [Line Items]
Fair Value	(1,334,000)	(1,290,000)
Notional Amount	231,651,000	303,394,000
Derivative Assets
Derivative [Line Items]
Fair Value	14,709,000	35,701,000
Notional Amount	2,214,207,000	6,659,862,000
Derivative Assets | Interest rate swaps
Derivative [Line Items]
Fair Value	78,000	17,095,000
Notional Amount	25,000,000	1,399,000,000
Derivative Assets | TBAs
Derivative [Line Items]
Fair Value	402,000	5,755,000
Notional Amount	140,000,000	2,445,000,000
Derivative Assets | Interest rate futures
Derivative [Line Items]
Fair Value	63,000	777,000
Notional Amount	40,000,000	213,700,000
Derivative Assets | Swaptions
Derivative [Line Items]
Fair Value	3,393,000	1,925,000
Notional Amount	285,000,000	1,065,000,000
Derivative Assets | Loan purchase and interest rate lock commitments
Derivative [Line Items]
Fair Value	10,773,000	10,149,000
Notional Amount	$ 1,724,207,000	$ 1,537,162,000