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Income taxes, income tax contingency (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Tax Contingency [Line Items]      
Unrecognized tax benefits, current $ 500,000 $ 318,000  
Unrecognized tax benefits, noncurrent 1,300,000 1,064,000  
Gross unrecognized tax benefits activity [Roll Forward]      
Gross unrecognized tax benefits, beginning balance 1,457,000 1,248,000 $ 1,121,000
Increases for tax positions for prior years 5,000 150,000 150,000
Decreases for tax positions for prior years 0 0 0
Increases for tax positions for current year 299,000 59,000 102,000
Settlements 0 0 0
Lapse of statute of limitations 0 0 (125,000)
Gross unrecognized tax benefits, ending balance $ 1,761,000 $ 1,457,000 $ 1,248,000
U.S. Federal Tax Authority [Member] | Minimum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2011    
Federal income tax year selected for audit 2011    
U.S. Federal Tax Authority [Member] | Maximum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2015    
Federal income tax year selected for audit 2012    
United Kingdom Tax Authority [Member] | Minimum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2007    
United Kingdom Tax Authority [Member] | Maximum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2015