Derivatives - Interest Rate Swap Contracts, Swaptions and Futures Contracts (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Weighted Average Remaining Swap / Option Term (Months)		63 months
Weighted Average Rate		1.60%
Notional Amount (3)	$ 16,275,000,000	$ 13,030,000,000
Asset Fair Value	32,609,000	60,518,000
Liability Fair Value	$ (107,954,000)	$ (137,393,000)
Interest Rate Swap 0-12 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum
Remaining / Underlying Term-Maximum	12 months	12 months
Weighted Average Remaining Swap / Option Term (Months)	8 months	5 months
Weighted Average Rate	1.16%	1.13%
Notional Amount (3)	$ 2,725,000,000	$ 920,000,000
Asset Fair Value	0	0
Liability Fair Value	$ (26,269,000)	$ (9,349,000)
Interest Rate Swap 13-24 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	13 months	13 months
Remaining / Underlying Term-Maximum	24 months	24 months
Weighted Average Remaining Swap / Option Term (Months)	13 months	16 months
Weighted Average Rate	1.24%	1.23%
Notional Amount (3)	$ 550,000,000	$ 2,900,000,000
Asset Fair Value	0	0
Liability Fair Value	$ (10,939,000)	$ (54,396,000)
Interest Rate Swap 25-36 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	25 months	25 months
Remaining / Underlying Term-Maximum	36 months	36 months
Weighted Average Remaining Swap / Option Term (Months)	30 months	31 months
Weighted Average Rate	0.99%	0.63%
Notional Amount (3)	$ 1,050,000,000	$ 350,000,000
Asset Fair Value	0	2,083,000
Liability Fair Value	$ (1,439,000)	$ 0
Interest Rate Swap 37-48 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum		37 months
Remaining / Underlying Term-Maximum		48 months
Weighted Average Remaining Swap / Option Term (Months)		37 months
Weighted Average Rate		1.00%
Notional Amount (3)		$ 300,000,000
Asset Fair Value		2,090,000
Liability Fair Value		$ 0
Interest Rate Swap 49-60 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	49 months	49 months
Remaining / Underlying Term-Maximum	60 months	60 months
Weighted Average Remaining Swap / Option Term (Months)	53 months	59 months
Weighted Average Rate	1.47%	1.56%
Notional Amount (3)	$ 2,350,000,000	$ 2,000,000,000
Asset Fair Value	2,871,000	0
Liability Fair Value	$ (481,000)	$ (7,414,000)
Interest Rate Swap 61-72 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum		61 months
Remaining / Underlying Term-Maximum		72 months
Weighted Average Remaining Swap / Option Term (Months)		61 months
Weighted Average Rate		1.48%
Notional Amount (3)		$ 300,000,000
Asset Fair Value		2,921,000
Liability Fair Value		$ 0
Interest Rate Swap 73-84 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	73 months
Remaining / Underlying Term-Maximum	84 months
Weighted Average Remaining Swap / Option Term (Months)	81 months
Weighted Average Rate	2.05%
Notional Amount (3)	$ 1,025,000,000
Asset Fair Value	1,163,000
Liability Fair Value	$ (1,679,000)
Interest Rate Swap 85-96 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	85 months	85 months
Remaining / Underlying Term-Maximum	96 months	96 months
Weighted Average Remaining Swap / Option Term (Months)	92 months	91 months
Weighted Average Rate	2.13%	1.47%
Notional Amount (3)	$ 1,625,000,000	$ 2,450,000,000
Asset Fair Value	1,088,000	50,650,000
Liability Fair Value	$ (1,581,000)	$ 0
Interest Rate Swap 97-108 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum		97 months
Remaining / Underlying Term-Maximum		108 months
Weighted Average Remaining Swap / Option Term (Months)		98 months
Weighted Average Rate		2.08%
Notional Amount (3)		$ 2,800,000,000
Asset Fair Value		2,774,000
Liability Fair Value		$ (7,534,000)
Interest Rate Swap 109-120 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	109 months
Remaining / Underlying Term-Maximum	120 months
Weighted Average Remaining Swap / Option Term (Months)	114 months
Weighted Average Rate	2.66%
Notional Amount (3)	$ 1,000,000,000
Asset Fair Value	0
Liability Fair Value	$ (55,433,000)
Interest Rate Swap 121-132 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	121 months	121 months
Remaining / Underlying Term-Maximum	132 months	132 months
Weighted Average Remaining Swap / Option Term (Months)	129 months	120 months
Weighted Average Rate	2.29%	2.66%
Notional Amount (3)	$ 2,350,000,000	$ 1,000,000,000
Asset Fair Value	27,476,000	0
Liability Fair Value	$ (191,000)	$ (58,520,000)
Basis Swap Contracts 0-60 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum
Remaining / Underlying Term-Maximum	60 months
Weighted Average Remaining Swap / Option Term (Months)	28 months
Weighted Average Rate	0.22%
Notional Amount (3)	$ 2,000,000,000
Asset Fair Value	11,000
Liability Fair Value	(327,000)
TBA Agency Securities
Derivative [Line Items]
Notional Amount (3)	1,600,000,000
Asset Fair Value	0
Liability Fair Value	(9,615,000)
Futures Contracts 0-15 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum
Remaining / Underlying Term-Maximum		15 months
Weighted Average Remaining Swap / Option Term (Months)		9 months
Weighted Average Rate		2.11%
Notional Amount (3)		$ 10,000,000
Asset Fair Value		0
Liability Fair Value		$ (180,000)
Interest rate swap contracts
Derivative [Line Items]
Notional amount, becoming effective in next twelve months	$ 6,375,000,000