Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 2,223,257,000	$ 2,158,010,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	2,223,257	2,158,010
Margin Deposit Assets	15,150,000	8,483,000
Interest Rate Swaps	(5,560,000)
TBA Contracts	(313,000)
Interest Rate Swaption		669,000
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Margin Deposit Assets	15,150,000	8,483,000
Interest Rate Swaps	0
TBA Contracts	0
Interest Rate Swaption		0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	2,223,257,000	2,158,010,000
Margin Deposit Assets	0	0
Interest Rate Swaps	(5,560,000)
TBA Contracts	(313,000)
Interest Rate Swaption		669,000
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Margin Deposit Assets	0	0
Interest Rate Swaps	0
TBA Contracts	$ 0
Interest Rate Swaption		$ 0