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Debt - Line of Credit (Details) - Fourth Amended and Restated Credit Agreement [Member] - Line of Credit [Member] - USD ($)
Dec. 19, 2014
Sep. 30, 2017
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity $ 400,000,000  
Percentage of loan availability from pledged assets that cannot exceed availability from mortgaged assets 10.00%  
Line of credit   $ 0
Letters of credit issued   64,400,000
Applicable Margin, Less Than Or Equal To 35% Utilization [Member]    
Credit Facilities [Line Items]    
Percentage of utilization 35.00%  
Applicable Margin, Greater Than 50% Utilization [Member]    
Credit Facilities [Line Items]    
Percentage of utilization 50.00%  
Unused Commitment Fee, Outstanding Debt Percentage Greater Than Or Equal To 50% [Member]    
Credit Facilities [Line Items]    
Quarterly commitment fee 0.25%  
Outstanding debt percentage 50.00%  
Unused Commitment Fee, Outstanding Debt Percentage Less Than 50% [Member]    
Credit Facilities [Line Items]    
Quarterly commitment fee 0.35%  
Outstanding debt percentage 50.00%  
Minimum [Member] | Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization [Member]    
Credit Facilities [Line Items]    
Percentage of utilization 35.00%  
Maximum [Member] | Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization [Member]    
Credit Facilities [Line Items]    
Percentage of utilization 50.00%  
LIBOR [Member] | Applicable Margin, Less Than Or Equal To 35% Utilization [Member]    
Credit Facilities [Line Items]    
Basis spread on variable rate basis 2.50%  
LIBOR [Member] | Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization [Member]    
Credit Facilities [Line Items]    
Basis spread on variable rate basis 3.25%  
LIBOR [Member] | Applicable Margin, Greater Than 50% Utilization [Member]    
Credit Facilities [Line Items]    
Basis spread on variable rate basis 3.50%  
LIBOR [Member] | Minimum [Member]    
Credit Facilities [Line Items]    
Basis spread on variable rate basis 2.50%  
LIBOR [Member] | Maximum [Member]    
Credit Facilities [Line Items]    
Basis spread on variable rate basis 3.50%  
Revolving Credit Facility [Member]    
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity $ 400,000,000  
Letter of credit sublimit [Member]    
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity 50,000,000  
Letters of credit issued   38,500,000
Swingline Line of Credit [Member]    
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity 50,000,000  
Option to increase maximum borrowing capacity [Member]    
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity $ 250,000,000  
Letter of Credit [Member]    
Credit Facilities [Line Items]    
Credit facility, maximum borrowing capacity   $ 64,500,000