Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 4,845	$ 6,563
Derivative liabilities	439	1,218
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	5,302,926	3,749,464
Assets, Fair Value Disclosure	5,307,771	3,756,027
Financial Liabilities Fair Value Disclosure	439	1,218
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Assets, Fair Value Disclosure	928	0
Financial Liabilities Fair Value Disclosure	0	1,218
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	5,301,118	3,747,190
Assets, Fair Value Disclosure	5,305,035	3,753,753
Financial Liabilities Fair Value Disclosure	439	0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	1,808	2,274
Assets, Fair Value Disclosure	1,808	2,274
Financial Liabilities Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	324
Fair Value, Measurements, Recurring | Interest rate swaps | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Fair Value, Measurements, Recurring | Interest rate swaps | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	324
Fair Value, Measurements, Recurring | Interest rate swaps | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Fair Value, Measurements, Recurring | Interest rate swaption
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	1,851	0
Fair Value, Measurements, Recurring | Interest rate swaption | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Fair Value, Measurements, Recurring | Interest rate swaption | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	1,851	0
Fair Value, Measurements, Recurring | Interest rate swaption | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Fair Value, Measurements, Recurring | TBA securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	2,066	6,239
Derivative liabilities	439	0
Fair Value, Measurements, Recurring | TBA securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | TBA securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	2,066	6,239
Derivative liabilities	439	0
Fair Value, Measurements, Recurring | TBA securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	928	0
Derivative liabilities	0	1,218
Fair Value, Measurements, Recurring | Futures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	928	0
Derivative liabilities	0	1,218
Fair Value, Measurements, Recurring | Futures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Futures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liabilities	$ 0	$ 0