Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Treasuries	$ 0	$ 146,530
Derivative asset	6,563	2,940
Derivative liabilities	1,218	269
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,749,464	3,026,989
U.S. Treasuries	0	146,530
Assets, Fair Value Disclosure	3,756,027	3,176,459
Financial Liabilities Fair Value Disclosure	1,218	269
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
U.S. Treasuries	0	146,530
Assets, Fair Value Disclosure	0	147,196
Financial Liabilities Fair Value Disclosure	1,218	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,747,190	3,019,746
U.S. Treasuries	0	0
Assets, Fair Value Disclosure	3,753,753	3,022,020
Financial Liabilities Fair Value Disclosure	0	269
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	2,274	7,243
U.S. Treasuries	0	0
Assets, Fair Value Disclosure	2,274	7,243
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	324	791
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	324	791
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 | Eurodollar futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	666
Fair Value, Measurements, Recurring | Eurodollar futures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	666
Fair Value, Measurements, Recurring | Eurodollar futures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Fair Value, Measurements, Recurring | Eurodollar futures | 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	6,239	1,483
Derivative liabilities	0	269
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	6,239	1,483
Derivative liabilities	0	269
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 | U.S. Treasury futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	1,218	0
Fair Value, Measurements, Recurring | U.S. Treasury futures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	1,218	0
Fair Value, Measurements, Recurring | U.S. Treasury futures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | U.S. Treasury futures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	$ 0	$ 0