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Income Taxes - Narrative (Details)
$ in Billions
 3 Months Ended
Mar. 31, 2018
USD ($)
Minimum  
Income Taxes  
Number of tax years open for state tax audit (in years) 3 years
Maximum  
Income Taxes  
Number of tax years open for state tax audit (in years) 5 years
Domestic Tax Authority  
Income Taxes  
Operating loss carryforwards subject to limitation $ 1.9
Domestic Tax Authority | Earliest Tax Year  
Income Taxes  
Open tax year 2014
Net Operating Loss And Other Carryforwards | Earliest Tax Year  
Income Taxes  
Open tax year 2005
Net Operating Loss And Other Carryforwards | Latest Tax Year  
Income Taxes  
Open tax year 2013