Accounts Receivable - Company's Accounts Receivable and Securitized Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 2,760,661	$ 2,371,778
Less allowance for doubtful accounts	(64,663)	(59,963)	$ (52,022)	$ (46,031)
Net accounts and securitized accounts receivable	2,695,998	2,311,815
Gross domestic accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	849,902	668,154
Gross domestic securitized accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	992,000	886,000
Gross foreign receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 918,759	$ 817,624