Fair Value of Financial Instruments - Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 1,082,650	$ 923,735
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	1,083,076	924,195
Cash and Overnight Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	513,328	427,307
Cash and Overnight Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	513,328	427,307
Corporate Commercial Paper | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	530,565	467,300
Corporate Commercial Paper | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	530,991	467,760
Time Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	38,757	29,128
Time Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 38,757	$ 29,128