Debt and Capital Lease Obligations - Asset Backed Notes (Details) - USD ($)
 3 Months Ended 12 Months Ended
Feb. 15, 2018
Dec. 20, 2017
Apr. 19, 2017
Apr. 30, 2018
Jan. 31, 2018
Debt Instrument [Line Items]          
Monthly fee percentage on outstanding balance         4.75%
Original Principal Amount       $ 1,094,276,000  
Original Net Proceeds       1,075,513,000  
Current Principal Amount       582,919,000  
2017-A Class A Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       313,220,000  
Original Net Proceeds     $ 304,451,000    
Current Principal Amount       $ 4,171,000  
Fixed Interest Rate       2.73%  
Effective interest rate       5.33%  
2017-A Class A Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 4,171,000 $ 59,794,000
2017-A Class B Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       106,270,000  
Original Net Proceeds     103,300,000    
Current Principal Amount       $ 106,270,000  
Fixed Interest Rate       5.11%  
Effective interest rate       5.73%  
2017-A Class B Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 106,270,000 106,270,000
2017-A Class C Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       50,340,000  
Original Net Proceeds     $ 48,919,000    
Current Principal Amount       $ 50,340,000  
Fixed Interest Rate       7.40%  
Effective interest rate       7.77%  
2017-A Class C Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 50,340,000 50,340,000
2017-B Class A Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       361,400,000  
Original Net Proceeds   $ 358,945,000      
Current Principal Amount       $ 188,997,000  
Fixed Interest Rate       2.73%  
Effective interest rate       4.95%  
2017-B Class A Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 188,997,000 292,663,000
2017-B Class B Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       132,180,000  
Original Net Proceeds   131,281,000      
Current Principal Amount       $ 132,180,000  
Fixed Interest Rate       4.52%  
Effective interest rate       5.14%  
2017-B Class B Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 132,180,000 132,180,000
2017-B Class C Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount       78,640,000  
Original Net Proceeds   $ 77,843,000      
Current Principal Amount       $ 78,640,000  
Fixed Interest Rate       5.95%  
Effective interest rate       6.29%  
2017-B Class C Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 78,640,000 78,640,000
Warehouse Notes [Member]          
Debt Instrument [Line Items]          
Original Principal Amount $ 52,200,000.0     52,226,000  
Original Net Proceeds 50,774,000        
Current Principal Amount       $ 22,321,000 0
Fixed Interest Rate       2.75%  
Effective interest rate       7.72%  
2016-B Class B Notes [Member] | Secured Debt [Member]          
Debt Instrument [Line Items]          
Current Principal Amount       $ 0 $ 73,589,000
Redemption amount 73,600,000        
Payments to redeem debt 50,300,000        
Adjustments $ 23,300,000        
Write off debt issuance costs       $ 400,000  
Asset-backed Securities [Member]          
Debt Instrument [Line Items]          
Monthly fee percentage on outstanding balance       4.75% 4.75%