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Short-term debt (Narrative) (Details)
Nov. 01, 2018
USD ($)
Aug. 03, 2018
USD ($)
Mar. 21, 2019
USD ($)
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Line of Credit Facility [Line Items]                              
Debt to EBITDA ratio (greater than)       3.0                      
Ratio of net indebtedness to EBITDA, debt covenant       4.07 3.83 4.02 4.05 3.80 4.02 4.05 4.12 4.21 4.08 4.40 4.80
Revolving Credit Facility | Bank Of America, N.A.                              
Line of Credit Facility [Line Items]                              
Line of credit facility, maximum borrowing amount   $ 25,000,000                          
Revolving Credit Facility | JP Morgan Chase Bank, N.A.                              
Line of Credit Facility [Line Items]                              
Line of credit facility, maximum borrowing amount $ 25,000,000                            
Revolving Credit Facility | LIBOR | Bank Of America, N.A.                              
Line of Credit Facility [Line Items]                              
Debt instrument, basis spread on variable rate (as a percent)   2.40%                          
Revolving Credit Facility | LIBOR | JP Morgan Chase Bank, N.A.                              
Line of Credit Facility [Line Items]                              
Debt instrument, basis spread on variable rate (as a percent) 2.25%                            
Line of Credit | Revolving Credit Facility                              
Line of Credit Facility [Line Items]                              
Ratio of net indebtedness to EBITDA, debt covenant       0.97                      
Subsequent Event | Revolving Credit Facility | JP Morgan Chase Bank, N.A.                              
Line of Credit Facility [Line Items]                              
Amount due     $ 0