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Debt (Narrative) (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Oct. 24, 2017
Jan. 31, 2017
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 300,000,000     $ 400,000,000
Line of credit facility, accordion feature 100,000,000      
Debt repayment 500,000,000 $ 337,576,000    
Debt issuance costs 9,091,000 $ 2,675,000    
Revolving Credit Facility [Member] | Subsequent Event [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity     $ 300,000,000  
Current borrowing capacity     $ 285,600,000  
Unsecured Senior Notes Due 2019 [Member]        
Debt Instrument [Line Items]        
Debt repayment $ 500,000,000      
Stated interest rate on unsecured senior notes 6.375%      
Unsecured Senior Notes Due 2021 [Member]        
Debt Instrument [Line Items]        
Senior Notes $ 800,000,000      
Stated interest rate on unsecured senior notes 7.125%      
Unsecured Senior Notes Due 2024 [Member]        
Debt Instrument [Line Items]        
Debt issuance $ 500,000,000      
Stated interest rate on unsecured senior notes 7.75%      
Debt issuance costs $ 8,900,000