Fair Value of Financial Instruments - Carrying Value and Fair Value of Long-Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	$ 647,748	$ 483,330
Fair Value	648,832	487,724
Revolving Credit Facility Due Mar. 2020 And Nov. 2016 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	345,962	171,200
Fair Value	345,962	171,200
5.375% Notes, Due Oct. 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	250,000	250,000
Fair Value	250,633	253,823
2.40% Term Loan, Due Jun. 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	6,201	7,710
Fair Value	6,332	7,889
2.05% Term Loan, Due Mar. 2023 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	27,802	33,607
Fair Value	28,206	34,122
1.30% Term Loan, Due Jun. 2023 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	7,855	9,423
Fair Value	7,825	9,370
1.55% Term Loan, Due Sep. 2025 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	9,928	11,390
Fair Value	$ 9,874	$ 11,320