Derivatives (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended						3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended									12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Hedging instrument [Member]	Dec. 31, 2013 Not Designated as Hedging Instrument [Member]	Dec. 31, 2012 Not Designated as Hedging Instrument [Member]	Mar. 31, 2011 Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap [Member]	Dec. 31, 2012 Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap [Member] Not Designated as Hedging Instrument [Member]	Mar. 31, 2011 Eurodollar Future [Member]	Dec. 31, 2013 Eurodollar Future [Member]	Dec. 31, 2012 Eurodollar Future [Member]	Dec. 31, 2013 Eurodollar Future [Member] Not Designated as Hedging Instrument [Member]	Dec. 31, 2013 Forward Contracts [Member]	Dec. 31, 2013 Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member] Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member] Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member] Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member] Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap Agreements, Maturities in Next 109-127 Months [Member] Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets	$ 18,488	$ 0			$ 18,488
Derivative Asset, Notional Amount					575,000
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value						(2,724)				1,336				(5,345)
Derivative Liabilities	6,681	42,537			(6,681)
Derivative Liability, Notional Amount		1,462,000		1,435,000	9,215,000	27,000				215,000				9,000,000
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value				(39,813)
Derivative, Notional Amount									1,462,000	790,000			0	9,000,000	150,000	185,000	15,000	385,000	25,000	180,000
Description of Derivative Activity Volume	The following table summarizes activity related to derivative instruments for the periods indicated:(amounts in thousands)Interest Rate Swaps Eurodollar FuturesNotional amount as of December 31, 2012:$1,462,000 $—Additions:380,000 20,250,000Settlements, terminations, or expirations:(1,052,000) (11,250,000)Notional amount as of December 31, 2013:(1)$790,000 $9,000,000(1)The Eurodollar futures notional amount as of December 31, 2013 represents the total notional of the 3-month contracts with expiration dates from 2016 to 2020. The maximum notional outstanding for any future 3-month period does not exceed $1,175,000.
Notional Amount of Derivative Instruments Added												20,250,000									380,000
Notional Amount of Derivative Instruments Terminated												(11,250,000)									1,052,000
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net							(2,825)	9,315	(908)		0	(19,391)	0
Loss on derivative instruments, net	$ (10,076)	$ (908)	$ (2,825)
Weighted-average Fixed Rate Swapped								1.56%							2.17%	0.92%	2.17%	1.58%	1.61%	2.13%