Derivatives (Details) - Interest Rate Swap Contracts, Swaptions and Futures Contracts (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Weighted Average Remaining Swap / Option Term (Months)	43 months		47 months
Weighted Average Rate	1.99%		1.98%
Notional Amount	$ 15,305,000,000		$ 16,025,000,000
Asset Fair Value	169,877,000	[1]	508,988,000	[1]
Liability Fair Value	(70,472,000)	[1]	(102,795,000)	[1]
Swap | Interest Rate Swap 0-12 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	0 months		0 months
Remaining / Underlying Term-Maximum	12 months		12 months
Weighted Average Remaining Swap / Option Term (Months)	9 months		2 months
Weighted Average Rate	1.43%		1.14%
Notional Amount	555,000,000.0		200,000,000
Asset Fair Value	0	[1]	0	[1]
Liability Fair Value	(7,312,000)	[1]	(2,089,000)	[1]
Swap | 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)	20 months		17 months
Weighted Average Rate	1.16%		1.13%
Notional Amount	2,725,000,000.0		920,000,000
Asset Fair Value	0	[1]	0	[1]
Liability Fair Value	(47,964,000)	[1]	(18,095,000)	[1]
Swap | 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)	25 months		28 months
Weighted Average Rate	1.24%		1.23%
Notional Amount	550,000,000.0		2,900,000,000
Asset Fair Value	0	[1]	0	[1]
Liability Fair Value	(14,557,000)	[1]	(81,108,000)	[1]
Swap | Interest Rate Swap 37-48 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	37 months		37 months
Remaining / Underlying Term-Maximum	48 months		48 months
Weighted Average Remaining Swap / Option Term (Months)	40 months		43 months
Weighted Average Rate	0.80%		0.63%
Notional Amount	650,000,000.0		350,000,000
Asset Fair Value	2,440,000	[1]	2,614,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | 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)	0 months		49 months
Weighted Average Rate	0.00%		1.00%
Notional Amount	0		300,000,000
Asset Fair Value	0	[1]	3,817,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | Interest Rate Swap 61-72 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	61 months		61 months
Remaining / Underlying Term-Maximum	72 months		72 months
Weighted Average Remaining Swap / Option Term (Months)	67 months		0 months
Weighted Average Rate	1.48%		0.00%
Notional Amount	300,000,000.0		0
Asset Fair Value	4,570,000	[1]	0	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | Interest Rate Swap 73-84 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	73 months		73 months
Remaining / Underlying Term-Maximum	84 months		84 months
Weighted Average Remaining Swap / Option Term (Months)	0 months		73 months
Weighted Average Rate	0.00%		1.48%
Notional Amount	0		300,000,000
Asset Fair Value	0	[1]	11,112,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | 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)	95 months		0 months
Weighted Average Rate	1.54%		0.00%
Notional Amount	550,000,000.0		0
Asset Fair Value	23,212,000	[1]	0	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | Interest Rate Swap 97-108 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	97 months		97 months
Remaining / Underlying Term-Maximum	108 months		108 months
Weighted Average Remaining Swap / Option Term (Months)	101 months		103 months
Weighted Average Rate	1.82%		1.47%
Notional Amount	4,700,000,000.0		2,450,000,000
Asset Fair Value	138,798,000	[1]	195,221,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | Interest Rate Swap 109-120 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	109 months		109 months
Remaining / Underlying Term-Maximum	120 months		120 months
Weighted Average Remaining Swap / Option Term (Months)	0 months		110 months
Weighted Average Rate	0.00%		2.08%
Notional Amount	0		2,800,000,000
Asset Fair Value	0	[1]	184,456,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swap | Interest Rate Swaptions 60 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	60 months		60 months
Swap | Interest Rate Swaptions 120 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	120 months		120 months
Future | Futures Contracts
Derivative [Line Items]
Remaining / Underlying Term-Minimum	0 months		0 months
Remaining / Underlying Term-Maximum	21 months		21 months
Weighted Average Remaining Swap / Option Term (Months)	11 months		13 months
Weighted Average Rate	2.13%		1.97%
Notional Amount	25,000,000.0		55,000,000
Asset Fair Value	0	[1]	0	[1]
Liability Fair Value	(639,000)	[1]	(1,503,000)	[1]
Swaption | Interest Rate Swaptions 60 Months
Derivative [Line Items]
Weighted Average Remaining Swap / Option Term (Months)	3 months		9 months
Weighted Average Rate	2.73%		2.73%
Notional Amount	4,000,000,000.0		4,000,000,000
Asset Fair Value	765,000	[1]	35,937,000	[1]
Liability Fair Value	0	[1]	0	[1]
Swaption | Interest Rate Swaptions 120 Months
Derivative [Line Items]
Weighted Average Remaining Swap / Option Term (Months)	3 months		6 months
Weighted Average Rate	3.63%		3.16%
Notional Amount	1,250,000,000.0		1,750,000,000
Asset Fair Value	92,000	[1]	75,831,000	[1]
Liability Fair Value	$ 0	[1]	$ 0	[1]

[1] See Note 5, “Fair Value of Financial Instruments” for additional discussion.