Debt (Details Textual)	6 Months Ended
	Jun. 30, 2015 USD ($) Institutions	May. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Number of lending institutions | Institutions	12
Maximum borrowing capacity	$ 1,200,000,000.0
Annual facility fees, percentage	0.10%
Ratio of debt to consolidated net worth	3
Debt to consolidated tangible net worth ratio	209.00%
Line of credit remaining capacity	$ 847,600,000
Current maturities classified as long-term debt	338,400,000		$ 698,500,000
Trade receivables borrowings			60,000,000
Cash proceeds on financing transaction backed by a portion of revenue earning equipment	156,400,000
Total available proceeds under trade receivables purchase and sale program	$ 175,000,000
Number of days under trade receivables purchase and sale program	364 days
Letters of credit and surety bonds outstanding	$ 333,900,000		334,300,000
U.S commercial paper [Member]
Debt Instrument [Line Items]
Commercial paper classified as long term debt	$ 311,432,000		276,694,000
Unsecured Medium Term Notes Due May 2020 [Member]
Debt Instrument [Line Items]
Face amount of unsecured medium-term notes issued		$ 300,000,000
Debt repurchase price, percentage	101.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 75,000,000
Letter of credit outstanding amount	$ 0
Minimum [Member]
Debt Instrument [Line Items]
Annual facility fees, percentage	0.075%
Maximum [Member]
Debt Instrument [Line Items]
Annual facility fees, percentage	0.25%
Asset-backed U.S. obligations [Member]
Debt Instrument [Line Items]
Debt	$ 362,076,000		$ 218,137,000