Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Derivative assets, fair value						$ 5,727							$ 5,727
Derivative assets, aggregate notional amount	$ 460,000					440,000					$ 460,000		440,000
Derivative liabilities, fair value	(41,205)					(35,898)					(41,205)		(35,898)
Derivative liabilities, aggregate notional amount	9,220,000					17,085,000					9,220,000		17,085,000
Derivative assets	7,835					5,727					7,835		5,727
Beginning of period notional amount					$ 17,525,000						17,525,000
Notional Amount of Derivative Instruments Added											3,285,000
Notional Amount of Derivative Instruments Terminated											(11,130,000)
End of period notional amount	9,680,000					17,525,000					9,680,000		17,525,000
Loss on derivative instruments, net	17,854		$ (52,749)	$ 17,090	(25,323)	(21,739)		$ 4,842	$ (23,074)	$ (13,422)	(43,128)		(53,393)		$ (10,076)
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets, fair value	7,835					5,727					7,835		5,727
Derivative assets, aggregate notional amount	460,000					440,000					460,000		440,000
Derivative liabilities, fair value	(12,108)					(3,002)					(12,108)		(3,002)
Derivative liabilities, aggregate notional amount	2,920,000					485,000					2,920,000		485,000
Eurodollar Future [Member]
Derivative [Line Items]
Derivative assets, fair value	0					0					0		0
Derivative assets, aggregate notional amount	0	[1]				0	[2]				0	[1]	0	[2]
Derivative liabilities, fair value	(29,097)					(32,896)					(29,097)		(32,896)
Derivative liabilities, aggregate notional amount	6,300,000	[1]				16,600,000	[2]				6,300,000	[1]	16,600,000	[2]
Beginning of period notional amount					16,600,000						16,600,000
Notional Amount of Derivative Instruments Added											0
Notional Amount of Derivative Instruments Terminated											(10,300,000)
End of period notional amount	6,300,000					16,600,000					6,300,000		16,600,000
Loss on derivative instruments, net											(20,963)		(27,551)		(19,391)
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Beginning of period notional amount					275,000						275,000
Notional Amount of Derivative Instruments Added											150,000
Notional Amount of Derivative Instruments Terminated											0
End of period notional amount	425,000					275,000					425,000		275,000
Loss on derivative instruments, net											6,522		4,912		0
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 13-24 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 0										$ 0
Weighted-average Fixed Pay Rate	0.00%										0.00%
Weighted-average Fixed Receive Rate	0.00%										0.00%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 250,000										$ 250,000
Weighted-average Fixed Pay Rate	1.91%										1.91%
Weighted-average Fixed Receive Rate	1.91%										1.91%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 50,000										$ 50,000
Weighted-average Fixed Pay Rate	1.75%										1.75%
Weighted-average Fixed Receive Rate	1.75%										1.75%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 100,000										$ 100,000
Weighted-average Fixed Pay Rate	1.69%										1.69%
Weighted-average Fixed Receive Rate	1.69%										1.69%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 0										$ 0
Weighted-average Fixed Pay Rate	0.00%										0.00%
Weighted-average Fixed Receive Rate	0.00%										0.00%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 0										$ 0
Weighted-average Fixed Pay Rate	0.00%										0.00%
Weighted-average Fixed Receive Rate	0.00%										0.00%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 25,000										$ 25,000
Weighted-average Fixed Pay Rate	2.71%										2.71%
Weighted-average Fixed Receive Rate	2.71%										2.71%
Receive-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 0										$ 0
Weighted-average Fixed Pay Rate	0.00%										0.00%
Weighted-average Fixed Receive Rate	0.00%										0.00%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Beginning of period notional amount					$ 650,000						$ 650,000
Notional Amount of Derivative Instruments Added											3,135,000
Notional Amount of Derivative Instruments Terminated											(830,000)
End of period notional amount	$ 2,955,000					$ 650,000					2,955,000		650,000
Loss on derivative instruments, net											(28,687)		$ (30,754)		$ 9,315
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 13-24 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 1,435,000										$ 1,435,000
Weighted-average Fixed Pay Rate	1.18%										1.18%
Weighted-average Fixed Receive Rate	1.18%										1.18%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 160,000										$ 160,000
Weighted-average Fixed Pay Rate	1.37%										1.37%
Weighted-average Fixed Receive Rate	1.37%										1.37%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 685,000										$ 685,000
Weighted-average Fixed Pay Rate	1.71%										1.71%
Weighted-average Fixed Receive Rate	1.71%										1.71%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 75,000										$ 75,000
Weighted-average Fixed Pay Rate	1.77%										1.77%
Weighted-average Fixed Receive Rate	1.77%										1.77%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 125,000										$ 125,000
Weighted-average Fixed Pay Rate	1.98%										1.98%
Weighted-average Fixed Receive Rate	1.98%										1.98%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 0										$ 0
Weighted-average Fixed Pay Rate	0.00%										0.00%
Weighted-average Fixed Receive Rate	0.00%										0.00%
Pay-fixed interest rate swap [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member]
Derivative [Line Items]
End of period notional amount	$ 475,000										$ 475,000
Weighted-average Fixed Pay Rate	2.86%										2.86%
Weighted-average Fixed Receive Rate	2.86%										2.86%

[1] The Eurodollar futures aggregate notional amount represents the total notional of the 3-month contracts with expiration dates from 2017 to 2020. The maximum notional outstanding for any future 3-month period did not exceed $725,000 as of December 31, 2015

[2] 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 December 31, 2014.