Fair Value of Financial Instruments - Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 1,111,973	$ 1,230,491
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	1,112,445	1,231,343
Cash and Overnight Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	440,121	417,456
Cash and Overnight Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	440,121	417,456
Corporate Commercial Paper | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	635,697	775,504
Corporate Commercial Paper | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	636,169	776,356
Time Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	36,155	37,531
Time Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 36,155	$ 37,531