Pledged Assets - Assets Pledged to Counterparties (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	$ 2,404,113,000	$ 2,095,574,000
Accrued interest receivable	9,527,000	8,490,000
Restricted cash	16,881,000	12,730,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	8,709,000	7,019,000
Restricted cash	8,123,000	3,997,000
Total Assets Pledged To Counterparties	2,439,398,000	1,913,288,000
FHLB Advances [Member]
Pledged Assets [Line Items]
Accrued interest receivable		645,000
Restricted cash		0
Total Assets Pledged To Counterparties		192,195,000
Clearing Margin [Member]
Pledged Assets [Line Items]
Accrued interest receivable	284,000
Restricted cash	0
Total Assets Pledged To Counterparties	8,775,000
Derivative [Member]
Pledged Assets [Line Items]
Accrued interest receivable	0	0
Restricted cash	8,508,000	8,483,000
Total Assets Pledged To Counterparties	8,508,000	8,483,000
Insurance Capital [Member]
Pledged Assets [Line Items]
Accrued interest receivable	4,000	6,000
Restricted cash	250,000	250,000
Total Assets Pledged To Counterparties	1,489,000	2,008,000
Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Accrued interest receivable	8,997,000	7,670,000
Restricted cash	16,881,000	12,730,000
Total Assets Pledged To Counterparties	2,458,170,000	2,115,974,000
Mortgage Backed Securities Pass Through Certificates [Member] | Repurchase Agreement [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	2,356,027,000	1,833,020,000
Mortgage Backed Securities Pass Through Certificates [Member] | FHLB Advances [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		191,550,000
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	1,235,000	1,752,000
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	2,357,262,000	2,026,322,000
Mortgage Backed Securities Structured Certificates [Member] | Repurchase Agreement [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	66,539,000	69,252,000
Mortgage Backed Securities Structured Certificates [Member] | FHLB Advances [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral		0
Mortgage Backed Securities Structured Certificates [Member] | Clearing Margin [Member]
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	8,491,000
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	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	$ 75,030,000	$ 69,252,000