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Segment Information - Customers Accounted for 10% or More of Revenue (Detail)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Customer Concentration Risk [Member] | Sales [Member] | Central Banks [Member]        
Revenue, Major Customer [Line Items]        
Entity wide revenue major customers percentage 62.00% 67.00% 65.00% 67.00%