Fair Value of Financial Instruments - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FHLBI stock	$ 74,250	$ 74,250
Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans, held-for-investment	24,055	31,582
FHLBI stock	74,250	74,250
Total	98,305	105,832
Repurchase agreements	17,532,303	13,602,484
Secured loans	1,650,000	1,650,000
Total	19,182,303	15,252,484
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans, held-for-investment	24,397	31,826
FHLBI stock	74,250	74,250
Total	98,647	106,076
Repurchase agreements	17,534,344	13,602,050
Secured loans	1,650,000	1,650,000
Total	$ 19,184,344	$ 15,252,050