Short-Term Debt - Credit Facilities and Related Borrowings (Parenthetical) (Detail)		6 Months Ended	12 Months Ended
	Mar. 23, 2018	Jun. 30, 2018	Dec. 31, 2017
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date		Mar. 22, 2022	Mar. 22, 2022
3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date	Mar. 22, 2021	Mar. 22, 2021	Mar. 22, 2021
1-year Term Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date		Nov. 01, 2018	Nov. 01, 2018