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Accounting Policies (Narratives) (Details) - USD ($)
9 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Cash, Insured and Uninsured [Abstract]            
Cash, FDIC Insured Amount       $ 250,000 $ 250,000  
Cash, CDIC insured amount       $ 100,000 $ 100,000  
LIFO Method Related Items [Abstract]            
Percentage of LIFO inventory       96.00% 94.00%  
Inventory, LIFO reserve       $ 16,126,000 $ 14,340,000  
Effect of LIFO inventory liquidation on income       100,000    
Disposition of Property, Plant and Equipment            
Net amount of (gains) losses netted against depreciation expense       11,800,000 32,500,000 $ 98,200,000
Additional operating expenses       $ 23,100,000 23,900,000  
Schedule of rental trucks depreciation:            
Percentage reduction for year one, depreciation       16.00%    
Percentage reduction for year two, depreciation       13.00%    
Percentage reduction for year three, depreciation       11.00%    
Percentage reduction for year four, depreciation       9.00%    
Percentage reduction for year five, depreciation       8.00%    
Percentage reduction for year six, depreciation       7.00%    
Percentage reduction for year seven, depreciation       6.00%    
Salvage value percentage       15.00%    
Percentage reduction if straight line approach used each year for depreciation calculation       5.70%    
Real Estate Companies Disclosures [Abstract]            
Carrying value of surplus real estate       $ 53,800,000 66,200,000  
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]            
Self insurance reserve       408,500,000 399,200,000  
Accrued insurance, noncurrent       15,200,000 13,700,000  
Estimated insurance recoveries       22,000 100,000  
Marketing and Advertising Expense [Abstract]            
Advertising expense       $ 8,100,000 $ 8,700,000 $ 9,600,000
Policyholder Benefits and Claims Incurred [Abstract]            
Deferred sales inducements, net $ 21,200,000 $ 23,000,000        
Deferred sales inducements, amortization expense $ 3,700,000 $ 3,300,000 $ 3,000,000      
Engine and Transmission Overhaul [Member] | Minimum [Member]            
Property, Plant and Equipment, Net, by Type [Abstract]            
Property, plant and equipment, useful life       2 years    
Engine and Transmission Overhaul [Member] | Maximum [Member]            
Property, Plant and Equipment, Net, by Type [Abstract]            
Property, plant and equipment, useful life       20 years    
All Nonrental Equipment [Member] | Minimum [Member]            
Property, Plant and Equipment, Net, by Type [Abstract]            
Property, plant and equipment, useful life       3 years    
All Nonrental Equipment [Member] | Maximum [Member]            
Property, Plant and Equipment, Net, by Type [Abstract]            
Property, plant and equipment, useful life       55 years