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Debt, Net - Convertible Notes (Details) - Convertible Notes
6 Months Ended
Jun. 30, 2018
$ / shares
Jun. 30, 2017
USD ($)
Mar. 31, 2014
USD ($)
Jul. 31, 2013
USD ($)
2020 Convertible Notes        
Debt Instrument [Line Items]        
Debt issued | $       $ 172,500,000.0
Stated interest rate (as a percent) 3.00%     3.00%
Initial conversion price (USD per share) $ 45.72      
Closing stock price at date of issuance (in dollars per share) $ 33.35      
Conversion rate (shares per $1,000 principal amount) 21.8718      
2021 Convertible Notes        
Debt Instrument [Line Items]        
Debt issued | $     $ 161,000,000.0  
Stated interest rate (as a percent) 2.875%   2.875%  
Initial conversion price (USD per share) $ 59.39      
Closing stock price at date of issuance (in dollars per share) $ 47.51      
Conversion rate (shares per $1,000 principal amount) 16.8386      
2022 Convertible Notes        
Debt Instrument [Line Items]        
Debt issued | $   $ 150,000,000.0    
Stated interest rate (as a percent) 3.25% 3.25%    
Initial conversion price (USD per share) $ 45.57      
Closing stock price at date of issuance (in dollars per share) $ 35.05      
Conversion rate (shares per $1,000 principal amount) 21.9467