Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,262,017,000	$ 3,022,174,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,262,017,000	3,022,174,000
Margin Deposit Assets	13,220,000	9,865,000
Interest Rate Swaps	11,034,000	9,500,000
TBA Contracts	(2,236,000)	(1,117,000)
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	13,220,000	9,865,000
Interest Rate Swaps	0	0
TBA Contracts	0	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	3,262,017,000	3,022,174,000
Margin Deposit Assets	0	0
Interest Rate Swaps	11,034,000	9,500,000
TBA Contracts	(2,236,000)	(1,117,000)
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	0
TBA Contracts	$ 0	$ 0