Derivatives - Interest Rate Swap Contracts, Swaptions and Futures Contracts (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Amount	$ 8,575,000,000	$ 10,800,000,000
Derivatives, at fair value	1,665,000	999,000
Liability Fair Value	$ (298,685,000)	$ (233,301,000)
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)	19 months	19 months
Weighted Average Rate	0.80%	0.63%
Notional Amount	$ 650,000,000	$ 350,000,000
Derivatives, at fair value	0	265,000
Liability Fair Value	$ (2,532,000)	$ (87,000)
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)	25 months	27 months
Weighted Average Rate	1.29%	1.16%
Notional Amount	$ 400,000,000	$ 700,000,000
Derivatives, at fair value	0	192,000
Liability Fair Value	$ (5,666,000)	$ (1,633,000)
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)	44 months	47 months
Weighted Average Rate	1.47%	1.46%
Notional Amount	$ 2,350,000,000	$ 2,000,000,000
Derivatives, at fair value	0	0
Liability Fair Value	$ (71,913,000)	$ (18,120,000)
Interest Rate Swap 49-60 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum		49 months
Remaining / Underlying Term-Maximum		60 months
Weighted Average Remaining Swap / Option Term (Months)		49 months
Weighted Average Rate		1.53%
Notional Amount		$ 350,000,000
Derivatives, at fair value		0
Liability Fair Value		$ (3,085,000)
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)	70 months
Weighted Average Rate	2.04%
Notional Amount	$ 275,000,000
Derivatives, at fair value	0
Liability Fair Value	$ (18,190,000)
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)	80 months	75 months
Weighted Average Rate	2.10%	2.05%
Notional Amount	$ 1,900,000,000	$ 1,025,000,000
Derivatives, at fair value	0	0
Liability Fair Value	$ (101,528,000)	$ (26,047,000)
Interest Rate Swap 85-96 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum		85 months
Remaining / Underlying Term-Maximum		96 months
Weighted Average Remaining Swap / Option Term (Months)		86 months
Weighted Average Rate		2.11%
Notional Amount		$ 1,375,000,000
Derivatives, at fair value		0
Liability Fair Value		$ (23,543,000)
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)	119 months	108 months
Weighted Average Rate	2.27%	2.66%
Notional Amount	$ 750,000,000	$ 1,000,000,000
Derivatives, at fair value	0	0
Liability Fair Value	$ (70,939,000)	$ (92,927,000)
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)	123 months	123 months
Weighted Average Rate	2.50%	2.30%
Notional Amount	$ 250,000,000	$ 2,000,000,000
Derivatives, at fair value	0	0
Liability Fair Value	$ (27,917,000)	$ (67,858,000)
Basis Swap Contracts 0-60 Months
Derivative [Line Items]
Remaining / Underlying Term-Minimum	0 months	0 months
Remaining / Underlying Term-Maximum	60 months	60 months
Weighted Average Remaining Swap / Option Term (Months)	19 months	22 months
Weighted Average Rate	0.22%	0.22%
Notional Amount	$ 2,000,000,000	$ 2,000,000,000
Derivatives, at fair value	1,665,000	542,000
Liability Fair Value	0	(1,000)
Interest rate swap contracts
Derivative [Line Items]
Derivatives, at fair value	0	457,000
Liability Fair Value	(298,685,000)	$ (233,301,000)
Notional amount, becoming effective in next six months	$ 1,175,000,000