Notes Payable and Revolving Lines of Credit - (Details)	Oct. 14, 2016 USD ($) agency extension	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 600,000,000
Notes payable to trusts		3,240,938,000
Notes Payable [Member]
Debt Instrument [Line Items]
Notes payable to trusts		3,240,938,000	$ 2,782,300,000
Notes payable subject to recourse		$ 2,660,814,000
Credit Agreement [Member]
Debt Instrument [Line Items]
Debt instrument, term | extension	2
Costs incurred	$ 8,000,000
Credit Agreement [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	1,150,000,000.00
Increase limit	$ 1,500,000,000.0
Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.00%
Number of credit rating agencies | agency	2
Four Year Revolving Credit Facility [Member] | Revolving Credit Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 500,000,000
Five Year Term Loan [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	430,000,000
Seven Year Term Loan [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 220,000,000
Federal Funds Effective Swap Rate [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Eurodollar [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Minimum [Member] | Eurodollar [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%
Basis spread on variable rate, alternative rating	0.0085
Minimum [Member] | Base Rate [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.35%
Basis spread on variable rate, alternative rating	0
Maximum [Member] | Eurodollar [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Basis spread on variable rate, alternative rating	0.0245
Maximum [Member] | Base Rate [Member] | Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Basis spread on variable rate, alternative rating	0.0145