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Long-term Debt - 5.625% Senior Unsecured Notes (Details) - USD ($)
Jan. 19, 2017
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Interest rate   5.625%  
Senior notes | 5.875% senior subordinated notes due 2021      
Debt Instrument [Line Items]      
Interest rate     5.875%
Senior notes | 5.625% Notes due 2027      
Debt Instrument [Line Items]      
Debt principal amount $ 400,000,000    
Interest rate 5.625% 5.625%  
Redemption price 105.625%    
Percentage of principal that must remain outstanding following redemption, at least 60.00%    
Period from completion of equity offering, during which notes can be redeemed 180 days