Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Derivative [Line Items]
Derivative assets, fair value	$ 8,079			$ 5,727
Derivative liabilities, fair value	(57,168)			(35,898)
Notional amount of derivative instruments added	815,000
Notional amount of derivative instruments terminated	(205,000)
Derivatives, notional amount	18,135,000			17,525,000
Gain (loss) on derivative instruments, net	(25,323)		(13,422)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	510,000			440,000
Derivative liabilities, aggregate notional amount	17,625,000			17,085,000
Forward Contracts [Member]
Derivative [Line Items]
Derivatives, notional amount	425,000
Weighted-average Fixed Pay Rate	1.98%
Weighted-average Fixed Receive Rate	1.98%
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets, fair value	8,079			5,727
Derivative liabilities, fair value	(7,784)			(3,002)
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	510,000			440,000
Derivative liabilities, aggregate notional amount	1,025,000			485,000
Eurodollar Future [Member]
Derivative [Line Items]
Derivative assets, fair value	0
Derivative liabilities, fair value	(49,384)			(32,896)
Notional amount of derivative instruments added	0
Notional amount of derivative instruments terminated	0
Derivatives, notional amount	16,600,000			16,600,000
Gain (loss) on derivative instruments, net	(16,488)		(4,414)
Eurodollar Future [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	0	[1]
Derivative liabilities, aggregate notional amount	16,600,000	[1]		16,600,000	[1]
Long [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added	0
Notional amount of derivative instruments terminated	0
Derivatives, notional amount	275,000			275,000
Gain (loss) on derivative instruments, net	4,528		0
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	250,000
Weighted-average Fixed Pay Rate	1.91%
Weighted-average Fixed Receive Rate	1.91%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	25,000
Weighted-average Fixed Pay Rate	2.71%
Weighted-average Fixed Receive Rate	2.71%
Short [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added	815,000
Notional amount of derivative instruments terminated	(205,000)
Derivatives, notional amount	1,260,000			650,000
Gain (loss) on derivative instruments, net	(13,363)		(9,008)
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	385,000
Weighted-average Fixed Pay Rate	1.21%
Weighted-average Fixed Receive Rate	1.21%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	470,000
Weighted-average Fixed Pay Rate	1.74%
Weighted-average Fixed Receive Rate	1.74%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	25,000
Weighted-average Fixed Pay Rate	1.61%
Weighted-average Fixed Receive Rate	1.61%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	75,000
Weighted-average Fixed Pay Rate	1.77%
Weighted-average Fixed Receive Rate	1.77%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	125,000
Weighted-average Fixed Pay Rate	1.98%
Weighted-average Fixed Receive Rate	1.98%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	0
Weighted-average Fixed Pay Rate	0.00%
Weighted-average Fixed Receive Rate	0.00%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	$ 180,000
Weighted-average Fixed Pay Rate	2.07%
Weighted-average Fixed Receive Rate	2.07%

[1] The Eurodollar futures aggregate notional amount represents the total notional of the 3-month contracts with expiration dates from 2015 to 2020. The maximum notional outstanding for any future 3-month period did not exceed $1,300,000 as of March 31, 2015 or as of December 31, 2014.