Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Derivative [Line Items]
Derivative liabilities, fair value	$ (23,974)		$ (23,974)		$ (6,681)
Gain (loss) on derivative instruments, net	(23,074)	11,353	(36,496)	11,336
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative liabilities, aggregate notional amount	11,810,000		11,810,000		9,215,000
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets, fair value	5,237		5,237		18,488
Derivative liabilities, fair value	(2,835)		(2,835)		(1,336)
Notional amount of derivative instruments added			75,000
Derivatives, notional amount	700,000		700,000
Weighted-average Fixed Rate Swapped	1.57%		1.57%
Gain (loss) on derivative instruments, net	(11,694)	11,353	(20,702)	11,336
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	290,000		290,000		575,000
Derivative liabilities, aggregate notional amount	410,000		410,000		215,000
Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	185,000		185,000
Weighted-average Fixed Rate Swapped	0.92%		0.92%
Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	235,000		235,000
Weighted-average Fixed Rate Swapped	1.45%		1.45%
Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	25,000		25,000
Weighted-average Fixed Rate Swapped	1.61%		1.61%
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 Rate Swapped	2.24%		2.24%
Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 85-108 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	30,000		30,000
Weighted-average Fixed Rate Swapped	1.93%		1.93%
Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-127 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	150,000		150,000
Weighted-average Fixed Rate Swapped	2.17%		2.17%
Eurodollar Future [Member]
Derivative [Line Items]
Derivative liabilities, fair value	(21,139)		(21,139)		(5,345)
Notional amount of derivative instruments added			2,400,000
Gain (loss) on derivative instruments, net	(11,380)	0	(15,794)	0
Eurodollar Future [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative liabilities, aggregate notional amount	11,400,000		11,400,000		9,000,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments terminated			$ (165,000)