Accounts Receivable - Company's Accounts Receivable and Securitized Accounts Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 2,663,653		$ 2,371,778
Gross domestic securitized accounts receivable	942,000	[1]	886,000
Less allowance for doubtful accounts	(66,194)		(59,963)	$ (48,989)	$ (46,031)
Net accounts and securitized accounts receivable	2,597,459		2,311,815
Gross domestic accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	746,954		668,154
Gross foreign receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 974,699		$ 817,624

[1] Reflects the impact of the Company's adoption of ASU 2016-02 "Leases", on January 1, 2019, using the modified retrospective transition method. Under this method, financial results reported in periods prior to 2019 are unchanged. Refer to footnote 2.