Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Derivative [Line Items]
Derivative assets, fair value	$ 7,297			$ 7,297			$ 18,488
Derivative liabilities, fair value	(18,058)			(18,058)			(6,681)
Notional amount of derivative instruments added				7,950,000
Notional amount of derivative instruments terminated				(190,000)
Derivatives, notional amount	17,550,000			17,550,000			9,790,000
Gain (loss) on derivative instruments, net	4,842		(24,019)	(31,654)		(12,683)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	4,700,000			4,700,000			575,000
Derivative liabilities, aggregate notional amount	12,850,000			12,850,000			9,215,000
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets, fair value	6,958			6,958			18,488
Derivative liabilities, fair value	(437)			(437)			(1,336)
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	800,000			800,000			575,000
Derivative liabilities, aggregate notional amount	150,000			150,000			215,000
Eurodollar Future [Member]
Derivative [Line Items]
Derivative assets, fair value	339			339
Derivative liabilities, fair value	(17,621)			(17,621)			(5,345)
Notional amount of derivative instruments added				7,600,000
Notional amount of derivative instruments terminated				0
Derivatives, notional amount	16,600,000			16,600,000			9,000,000
Gain (loss) on derivative instruments, net	3,857		(17,794)	(11,937)		(17,794)
Eurodollar Future [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets, aggregate notional amount	3,900,000	[1]		3,900,000	[1]
Derivative liabilities, aggregate notional amount	12,700,000	[1]		12,700,000	[1]		9,000,000	[1]
Long [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added				275,000
Notional amount of derivative instruments terminated				0
Derivatives, notional amount	275,000			275,000			0
Gain (loss) on derivative instruments, net	339		0	339		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	250,000			250,000
Weighted-average Fixed Pay Rate	2.00%			2.00%
Weighted-average Fixed Receive Rate	2.00%			2.00%
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 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	3.00%			3.00%
Weighted-average Fixed Receive Rate	3.00%			3.00%
Short [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of derivative instruments added				75,000
Notional amount of derivative instruments terminated				(190,000)
Derivatives, notional amount	675,000			675,000			790,000
Gain (loss) on derivative instruments, net	646		(6,225)	(20,056)		5,111
Short [Member] | 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 Pay Rate	0.92%			0.92%
Weighted-average Fixed Receive Rate	0.92%			0.92%
Short [Member] | 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 Pay Rate	1.45%			1.45%
Weighted-average Fixed Receive Rate	1.45%			1.45%
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			25,000
Weighted-average Fixed Pay Rate	1.61%			1.61%
Weighted-average Fixed Receive Rate	1.61%			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	50,000			50,000
Weighted-average Fixed Pay Rate	2.20%			2.20%
Weighted-average Fixed Receive Rate	2.20%			2.20%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	30,000			30,000
Weighted-average Fixed Pay Rate	1.93%			1.93%
Weighted-average Fixed Receive Rate	1.93%			1.93%
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member]
Derivative [Line Items]
Derivatives, notional amount	$ 150,000			$ 150,000
Weighted-average Fixed Pay Rate	2.17%			2.17%
Weighted-average Fixed Receive Rate	2.17%			2.17%

[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 September 30, 2014 and $1,175,000 as of December 31, 2013.