Debt - Summary of Secured Structured Financings (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Balance	$ 15,783,587	$ 15,195,887
Secured Structured Financings
Debt Instrument [Line Items]
Balance	15,783,587	15,195,887
Initial Note Amounts Issued	35,859,736	33,127,485
Collateral	20,069,771	19,133,315
Restricted Cash	1,429,853	1,280,881
Secured Structured Financings | 2010 Securitizations
Debt Instrument [Line Items]
Balance	509,699	632,251
Initial Note Amounts Issued	4,671,749	4,671,749
Collateral	941,754	1,143,435
Restricted Cash	206,218	205,190
Secured Structured Financings | 2010 Securitizations | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2016-10	2016-10
Initial Weighted Average Interest Rate	1.04%	1.04%
Secured Structured Financings | 2010 Securitizations | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2017-11	2017-11
Initial Weighted Average Interest Rate	1.44%	1.44%
Secured Structured Financings | 2011 Securitizations
Debt Instrument [Line Items]
Balance	843,443	1,218,208
Initial Note Amounts Issued	5,605,609	5,605,609
Collateral	1,208,325	1,634,220
Restricted Cash	179,484	195,854
Secured Structured Financings | 2011 Securitizations | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2015-10	2015-10
Initial Weighted Average Interest Rate	1.21%	1.21%
Secured Structured Financings | 2011 Securitizations | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2017-09	2017-09
Initial Weighted Average Interest Rate	2.80%	2.80%
Secured Structured Financings | 2012 Securitizations
Debt Instrument [Line Items]
Balance	3,554,805	4,061,127
Initial Note Amounts Issued	8,023,840	8,023,840
Collateral	4,419,949	5,013,135
Restricted Cash	395,580	383,677
Secured Structured Financings | 2012 Securitizations | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2017-11	2017-11
Initial Weighted Average Interest Rate	0.92%	0.92%
Secured Structured Financings | 2012 Securitizations | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2018-12	2018-12
Initial Weighted Average Interest Rate	1.68%	1.68%
Secured Structured Financings | 2013 Securitizations
Debt Instrument [Line Items]
Balance	4,945,429	5,503,580
Initial Note Amounts Issued	6,689,700	6,689,700
Collateral	5,873,396	6,465,840
Restricted Cash	377,271	351,160
Secured Structured Financings | 2013 Securitizations | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2019-01	2019-01
Initial Weighted Average Interest Rate	0.89%	0.89%
Secured Structured Financings | 2013 Securitizations | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2021-01	2021-01
Initial Weighted Average Interest Rate	1.59%	1.59%
Secured Structured Financings | 2014 Securitizations
Debt Instrument [Line Items]
Balance	1,393,823
Initial Note Amounts Issued	1,500,000
Collateral	1,624,017
Restricted Cash	90,167
Secured Structured Financings | 2014 Securitizations | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2020-04
Initial Weighted Average Interest Rate	1.72%
Secured Structured Financings | Public Securitizations
Debt Instrument [Line Items]
Balance	11,247,199	11,415,166
Initial Note Amounts Issued	26,490,898	24,990,898
Collateral	14,067,441	14,256,630
Restricted Cash	1,248,720	1,135,881
Secured Structured Financings | 2010 Private Issuance
Debt Instrument [Line Items]
Balance	201,747	219,704
Initial Note Amounts Issued	516,000	516,000
Collateral	359,416	378,434
Restricted Cash	9,611	8,435
Secured Structured Financings | 2010 Private Issuance | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2011-06	2011-06
Initial Weighted Average Interest Rate	1.29%	1.29%
Secured Structured Financings | 2011 Private Issuances
Debt Instrument [Line Items]
Balance	554,257	662,138
Initial Note Amounts Issued	4,856,525	4,856,525
Collateral	796,959	908,304
Restricted Cash	39,216	36,449
Secured Structured Financings | 2011 Private Issuances | Minimum
Debt Instrument [Line Items]
Initial Weighted Average Interest Rate	1.46%	1.46%
Secured Structured Financings | 2011 Private Issuances | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2018-12	2018-12
Initial Weighted Average Interest Rate	1.80%	1.80%
Secured Structured Financings | 2012 Private Issuance
Debt Instrument [Line Items]
Balance	22,256	30,526
Initial Note Amounts Issued	70,308	70,308
Collateral	26,678	35,378
Restricted Cash	2,763	3,016
Secured Structured Financings | 2012 Private Issuance | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2016-05	2016-05
Initial Weighted Average Interest Rate	1.07%	1.07%
Secured Structured Financings | 2013 Private Issuance
Debt Instrument [Line Items]
Balance	2,552,886	2,868,353
Initial Note Amounts Issued	2,693,754	2,693,754
Collateral	3,275,012	3,554,569
Restricted Cash	94,111	97,100
Secured Structured Financings | 2013 Private Issuance | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2018-09	2018-09
Initial Weighted Average Interest Rate	1.13%	1.13%
Secured Structured Financings | 2013 Private Issuance | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2020-09	2020-09
Initial Weighted Average Interest Rate	1.38%	1.38%
Secured Structured Financings | 2014 Private Issuance
Debt Instrument [Line Items]
Balance	1,205,242
Initial Note Amounts Issued	1,232,251
Collateral	1,544,265
Restricted Cash	35,432
Secured Structured Financings | 2014 Private Issuance | Minimum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2018-03
Initial Weighted Average Interest Rate	1.15%
Secured Structured Financings | 2014 Private Issuance | Maximum
Debt Instrument [Line Items]
Original Estimated Maturity Dates	2021-06
Initial Weighted Average Interest Rate	1.40%
Secured Structured Financings | Privately Issued Amortizing Notes
Debt Instrument [Line Items]
Balance	4,536,388	3,780,721
Initial Note Amounts Issued	9,368,838	8,136,587
Collateral	6,002,330	4,876,685
Restricted Cash	$ 181,133	$ 145,000