Derivatives (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Derivative assets, fair value						$ 5,727
Derivative liabilities		$ (48,240)		$ (48,240)		(35,898)
Derivative liabilities, fair value						(35,898)
Derivative assets		20,804		20,804		5,727
Notional amount of derivative instruments added				1,785,000
Notional amount of derivative instruments terminated				(3,120,000)
Derivatives, notional amount		16,190,000		16,190,000		17,525,000
Gain (loss) on derivative instruments, net		17,090	$ (23,074)	(8,233)	$ (36,496)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount		1,305,000		1,305,000		440,000
Derivative liabilities, aggregate notional amount		14,885,000		14,885,000		17,085,000
Forward Contracts [Member]
Derivative [Line Items]
Derivatives, notional amount		$ 800,000		$ 800,000
Weighted-average Fixed Pay Rate		2.38%		2.38%
Weighted-average Fixed Receive Rate		2.38%		2.38%
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets, fair value		$ 20,804		$ 20,804		5,727
Derivative liabilities, fair value		(4,249)		(4,249)		(3,002)
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount		1,305,000		1,305,000		440,000
Derivative liabilities, aggregate notional amount		885,000		885,000		485,000
Eurodollar Future [Member]
Derivative [Line Items]
Derivative assets, fair value		0		0
Derivative liabilities, fair value		(43,991)		(43,991)		(32,896)
Notional amount of derivative instruments added				0
Notional amount of derivative instruments terminated				2,600,000
Derivatives, notional amount		14,000,000		14,000,000		16,600,000
Gain (loss) on derivative instruments, net		2,573	(11,380)	(13,915)	(15,794)
Eurodollar Future [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	[1]	0		0
Derivative liabilities, aggregate notional amount	[1]	14,000,000		14,000,000		16,600,000
Long [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added				50,000
Notional amount of derivative instruments terminated				0
Derivatives, notional amount		325,000		325,000		275,000
Gain (loss) on derivative instruments, net		(1,746)	0	2,782	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		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 300,000		$ 300,000
Weighted-average Fixed Pay Rate		1.24%		1.24%
Weighted-average Fixed Receive Rate		1.24%		1.24%
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
Derivatives, notional amount		$ 0		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 25,000
Weighted-average Fixed Pay Rate		2.71%		2.71%
Weighted-average Fixed Receive Rate		2.71%		2.71%
Short [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added				$ 1,735,000
Notional amount of derivative instruments terminated				(520,000)
Derivatives, notional amount		$ 1,865,000		1,865,000		$ 650,000
Gain (loss) on derivative instruments, net		16,263	$ (11,694)	2,900	$ (20,702)
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
Derivatives, notional amount		$ 345,000		$ 345,000
Weighted-average Fixed Pay Rate		1.13%		1.13%
Weighted-average Fixed Receive Rate		1.13%		1.13%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
Derivatives, notional amount		$ 835,000		$ 835,000
Weighted-average Fixed Pay Rate		1.77%		1.77%
Weighted-average Fixed Receive Rate		1.77%		1.77%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
Derivatives, notional amount		$ 0		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		0.00%
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		$ 75,000
Weighted-average Fixed Pay Rate		1.77%		1.77%
Weighted-average Fixed Receive Rate		1.77%		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		$ 125,000
Weighted-average Fixed Pay Rate		1.98%		1.98%
Weighted-average Fixed Receive Rate		1.98%		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		$ 0
Weighted-average Fixed Pay Rate		0.00%		0.00%
Weighted-average Fixed Receive Rate		0.00%		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		$ 485,000		$ 485,000
Weighted-average Fixed Pay Rate		2.96%		2.96%
Weighted-average Fixed Receive Rate		2.96%		2.96%

[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 June 30, 2015 or as of December 31, 2014.