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5. Notes Payable and Long-Term Debt (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2014
Dec. 31, 2010
Debt Instrument [Line Items]        
Debt and Capital Lease Obligations $ 435,000 $ 434,500    
Long-term Debt and Capital Lease Obligations, Maturities, Repayments of Principal Remainder of Fiscal Year 400 300    
Long-term Debt and Capital Lease Obligations 434,581 434,186    
Maturities of Long-term Debt [Abstract]        
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 400      
Long-term Debt, Maturities, Repayments of Principal in Year Two 300      
Long-term Debt, Maturities, Repayments of Principal in Year Three 425,300      
Long-term Debt, Maturities, Repayments of Principal in Year Four 300      
Long-term Debt, Maturities, Repayments of Principal in Year Five 300      
Long-term Debt, Maturities, Repayments of Principal after Year Five 10,300      
Line of Credit Facility, Remaining Borrowing Capacity 208,400      
Letters of Credit Outstanding, Amount 4,200      
Performance Guarantee [Member]        
Maturities of Long-term Debt [Abstract]        
Letters of Credit Outstanding, Amount 4,200      
Unsecured Debt [Member] | Senior Notes 4.875% due 2020 [Member]        
Debt Instrument [Line Items]        
Unsecured Debt 425,000 425,000    
Long-term debt 423,100 422,500   $ 422,600
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net 1,900 2,500    
Debt Instrument, Interest Rate, Effective Percentage       4.946%
Face amount of debt sold       $ 425,000
Debt Instrument, Interest Rate, Stated Percentage       4.875%
Capital Lease Obligations [Member] | Capital Leases and Other Debt [Member]        
Debt Instrument [Line Items]        
Long-term Debt and Capital Lease Obligations, Including Current Maturities 11,900 12,000    
Line of Credit [Member]        
Maturities of Long-term Debt [Abstract]        
Line of Credit Facility, Maximum Borrowing Capacity     $ 200,000  
Long-term Line of Credit $ 0      
Line of Credit Facility, Interest Rate at Period End 2.94%      
Standby Letters of Credit [Member]        
Maturities of Long-term Debt [Abstract]        
Letters of Credit Outstanding, Amount $ 500 $ 800