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Debt and Other Financing (Other Financing) (Details)
1 Months Ended 3 Months Ended
Feb. 21, 2019
EUR (€)
Jun. 30, 2015
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Mar. 31, 2013
Mar. 31, 2009
Mar. 31, 2008
Debt Instrument [Line Items]                
Amount outstanding under the revolving credit facility       $ 0        
Letters of credit outstanding       29,400,000 $ 37,700,000      
Revolving Credit Facility                
Debt Instrument [Line Items]                
Debt instrument term   5 years            
Maximum borrowing capacity   $ 400,000,000.0            
AIO                
Debt Instrument [Line Items]                
Letters of credit outstanding       $ 29,000,000        
6.50% Notes, due March 2019                
Debt Instrument [Line Items]                
Write off of debt issuance cost     $ 300,000          
Interest rate, stated percentage             6.50% 6.50%
4.60% Notes, due March 2020                
Debt Instrument [Line Items]                
Interest rate, stated percentage           4.60%    
Subsequent Event | Revolving Credit Facility                
Debt Instrument [Line Items]                
Debt instrument term 3 years              
Maximum borrowing capacity | € € 200,000,000.0              
Subsequent Event | LIBOR | Revolving Credit Facility | Line of Credit                
Debt Instrument [Line Items]                
Basis spread 2.25%              
Subsequent Event | EURIBOR | Revolving Credit Facility | Line of Credit                
Debt Instrument [Line Items]                
Basis spread 2.25%