Pledged Assets - Assets Pledged to Counterparties (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	$ 2,179,602,000	$ 2,095,574,000
Accrued interest receivable	8,792,000	8,490,000
Restricted cash	18,984,000	12,730,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	7,912,000	7,019,000
Restricted cash	3,584,000	3,997,000
Total Assets Pledged To Counterparties	2,189,345,000	1,913,288,000
FHLB Advances [Member]
Pledged Assets [Line Items]
Accrued interest receivable	0	645,000
Restricted cash	0	0
Total Assets Pledged To Counterparties	0	192,195,000
Derivative [Member]
Pledged Assets [Line Items]
Accrued interest receivable	0	0
Restricted cash	15,150,000	8,483,000
Total Assets Pledged To Counterparties	15,150,000	8,483,000
Insurance Capital [Member]
Pledged Assets [Line Items]
Accrued interest receivable	6,000	6,000
Restricted cash	250,000	250,000
Total Assets Pledged To Counterparties	2,009,000	2,008,000
Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Accrued interest receivable	7,918,000	7,670,000
Restricted cash	18,984,000	12,730,000
Total Assets Pledged To Counterparties	2,206,504,000	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	2,120,581,000	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	0	191,550,000
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	1,753,000	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	2,122,334,000	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	57,268,000	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	0
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	$ 57,268,000	$ 69,252,000