Fair Value of Financial Instruments - Carrying Value and Fair Value of Long-Term Debt (Parenthetical) (Detail)	Mar. 31, 2018	Dec. 31, 2017
2.40% Term Loan, Due Jun. 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percent on aggregate principal amount of outstanding	2.40%	2.40%
2.05% Term Loan, Due Mar. 2023 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percent on aggregate principal amount of outstanding	2.05%	2.05%
1.30% Term Loan, Due Jun. 2023 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percent on aggregate principal amount of outstanding	1.30%	1.30%
1.55% Term Loan, Due Sep. 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percent on aggregate principal amount of outstanding	1.55%	1.55%