Pledged Assets - Assets Pledged to Counterparties (Details) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Pledged Assets [Line Items]
Mortgage Backed Securities At Fair Value Pledged As Collateral	$ 1,918,265,000	$ 2,095,574,000
Accrued interest receivable	8,333,000	8,490,000
Restricted cash	17,778,000	12,730,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	7,550,000	7,019,000
Restricted cash	1,250,000	3,997,000
Total Assets Pledged To Counterparties	2,057,298,000	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	16,239,000	8,483,000
Total Assets Pledged To Counterparties	16,239,000	8,483,000
Insurance Capital [Member]
Pledged Assets [Line Items]
Accrued interest receivable	6,000	6,000
Restricted cash	289,000	250,000
Total Assets Pledged To Counterparties	2,051,000	2,008,000
Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Accrued interest receivable	7,556,000	7,670,000
Restricted cash	17,778,000	12,730,000
Total Assets Pledged To Counterparties	2,075,588,000	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	1,986,751,000	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	1,756,000	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	1,988,507,000	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	61,747,000	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	$ 61,747,000	$ 69,252,000