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Short-Term Debt (Narrative) (Details)
Nov. 01, 2017
USD ($)
Aug. 01, 2017
USD ($)
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Line of Credit Facility [Line Items]                            
Debt to EBITDA ratio (greater than)     3.0                      
Ratio of net indebtedness to EBITDA, debt covenant     3.80 4.02 4.05 4.12 4.21 4.08 4.40 4.80 4.40 4.56 4.61 4.62
Revolving Credit Facility [Member] | Bank Of America, N.A. [Member]                            
Line of Credit Facility [Line Items]                            
Line of credit facility, maximum borrowing amount   $ 25,000,000                        
Revolving Credit Facility [Member] | JPMorgan Chase Bank N.A. [Member]                            
Line of Credit Facility [Line Items]                            
Line of credit facility, maximum borrowing amount $ 25,000,000                          
Revolving Credit Facility [Member] | LIBOR [Member] | Bank Of America, N.A. [Member]                            
Line of Credit Facility [Line Items]                            
Debt instrument, basis spread on variable rate (as a percent)   2.50%                        
Revolving Credit Facility [Member] | LIBOR [Member] | JPMorgan Chase Bank N.A. [Member]                            
Line of Credit Facility [Line Items]                            
Debt instrument, basis spread on variable rate (as a percent) 2.25%                          
Line of Credit [Member] | Revolving Credit Facility [Member]                            
Line of Credit Facility [Line Items]                            
Ratio of net indebtedness to EBITDA, debt covenant     0.75