Fair Value of Financial Instruments - Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 1,230,491	$ 923,735
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	1,231,343	924,195
Cash and Overnight Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	417,456	427,307
Cash and Overnight Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	417,456	427,307
Corporate Commercial Paper | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	775,504	467,300
Corporate Commercial Paper | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	776,356	467,760
Time Deposits | Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	37,531	29,128
Time Deposits | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 37,531	$ 29,128