Accounts Receivable Securitization - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Aug. 20, 2018	Dec. 31, 2017	Mar. 08, 2017
Transfers and Servicing [Abstract]
Investment limit					$ 125,000,000		$ 100,000,000
Amount drawn on investment limit	$ 0		$ 0			$ 85,000,000
Proceeds from accounts receivable securitization			827,900,000	$ 719,700,000
Cash collection			803,400,000	579,300,000
Outstanding principal on receivables sold under securitization	157,300,000		157,300,000			$ 132,800,000
Loss related to receivables sold		$ 0		1,200,000
Accounts receivable securitization, program costs	$ 1,900,000	$ 700,000	$ 4,300,000	$ 2,900,000