Debt (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Nov. 30, 2012
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	$ 19,100		$ 19,100		$ 22,000
Amount outstanding	235,005		235,005		0
Short-term borrowings	30,504		30,504		19,623
Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	2,000,000		2,000,000		1,200,000
Outstanding borrowings on the revolving credit facility	235,000		235,000		0
Average outstanding borrowings	$ 334,700	$ 412,000	$ 296,700	$ 392,900
Pricing on the Facility based on market rates			LIBOR plus 1.00% to 2.05%
Description of Variable Rate Basis			LIBOR
Basis spread on variable rate (in hundredths)			1.00%
Line of credit, interest rate during period	1.20%	1.40%	1.10%	1.20%
Covenant terms, minimum leverage ratio through September 2014	350.00%		350.00%
Covenant Terms Minimum Leverage Ratio Through Specified Date Following a Material Acquisition	400.00%		400.00%
Covenant terms, minimum cash interest coverage ratio	300.00%		300.00%
Overdraft Facilities [Member]
Line of Credit Facility [Line Items]
Borrowing capacity remaining	$ 42,800		$ 42,800
Bank Overdrafts	$ 19,900		$ 19,900		$ 14,600
Long-Term Senior Notes [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount						$ 275,000
Debt Instrument, Interest Rate, Stated Percentage	4.40%		4.40%
Minimum [Member]
Line of Credit Facility [Line Items]
Description of Variable Rate Basis			LIBOR
Basis spread on variable rate (in hundredths)			1.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Description of Variable Rate Basis			LIBOR
Basis spread on variable rate (in hundredths)			2.05%