Pledged Assets - Assets Pledged to Counterparties (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	$ 3,450,947,000	$ 2,972,290,000
Accrued interest receivable	14,366,000	11,512,000
Restricted cash	19,865,000	20,950,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	13,719,000	10,623,000
Restricted cash	6,540,000	10,835,000
Total Assets Pledged To Counterparties	3,471,206,000	2,981,715,000
Clearing Margin [Member]
Pledged Assets [Line Items]
Accrued interest receivable		266,000
Restricted cash		0
Total Assets Pledged To Counterparties		11,234,000
Derivative [Member]
Pledged Assets [Line Items]
Accrued interest receivable	0	0
Restricted cash	13,325,000	9,865,000
Total Assets Pledged To Counterparties	13,325,000	9,865,000
Insurance Capital [Member]
Pledged Assets [Line Items]
Accrued interest receivable		4,000
Restricted cash		250,000
Total Assets Pledged To Counterparties		1,319,000
Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Accrued interest receivable	13,719,000	10,893,000
Restricted cash	19,865,000	20,950,000
Total Assets Pledged To Counterparties	3,484,531,000	3,004,133,000
Mortgage Backed Securities Pass Through Certificates [Member] | Repurchase Agreement [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	3,312,428,000	2,854,062,000
Mortgage Backed Securities Pass Through Certificates [Member] | Clearing Margin [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		0
Mortgage Backed Securities Pass Through Certificates [Member] | Derivative [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	0	0
Mortgage Backed Securities Pass Through Certificates [Member] | Insurance Capital [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		1,065,000
Mortgage Backed Securities Pass Through Certificates [Member] | Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	3,312,428,000	2,855,127,000
Mortgage Backed Securities Structured Certificates [Member] | Repurchase Agreement [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	138,519,000	106,195,000
Mortgage Backed Securities Structured Certificates [Member] | Clearing Margin [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		10,968,000
Mortgage Backed Securities Structured Certificates [Member] | Derivative [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	0	0
Mortgage Backed Securities Structured Certificates [Member] | Insurance Capital [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		0
Mortgage Backed Securities Structured Certificates [Member] | Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	$ 138,519,000	$ 117,163,000