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Income Taxes - Expiration of Loss and Tax Credit Carryforwards (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Operating Loss and Tax Credit Carryforward [Line Items]  
Loss Carryforwards $ 632,915
Tax Credit Carryforwards 222,353
Period of Expiration 2017 - 2021  
Operating Loss and Tax Credit Carryforward [Line Items]  
Loss Carryforwards 39,939
Tax Credit Carryforwards 7,085
Thereafter  
Operating Loss and Tax Credit Carryforward [Line Items]  
Loss Carryforwards 317,915
Tax Credit Carryforwards 193,867
No expiration date  
Operating Loss and Tax Credit Carryforward [Line Items]  
Loss Carryforwards 275,061
Tax Credit Carryforwards $ 21,401