Debt and lease obligations (short-term borrowings) (Details) - USD ($) $ in Thousands		1 Months Ended		12 Months Ended
		Mar. 31, 2015	Mar. 31, 2011	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt instruments [Line Items]
Short-term borrowings				$ 434,000	$ 160,000
Credit facility commitment				$ 525,000	$ 350,000
Credit facility, date of expiration				Feb. 21, 2019
Credit facility, amount outstanding, interest rate				1.89%	1.63%
Daily average amount outstanding				$ 270,063	$ 43,675	$ 0
Daily average amount outstanding, weighted-average interest rate				1.66%	1.63%
Outstanding letters of credit	[1]			$ (12,726)
Net available for borrowing as of December 31, 2015				$ 78,274
Minimum [Member]
Debt instruments [Line Items]
Commitment fee percentage				0.20%
Maximum [Member]
Debt instruments [Line Items]
Commitment fee percentage				0.40%
Short-term bank loan [Member]
Debt instruments [Line Items]
Debt issuance date		Mar. 05, 2015
Principal amount issued		$ 75,000
Debt maturity date, end		Mar. 03, 2016
Proceeds from loan, net of offering costs		$ 74,880
Short-term borrowings, daily average amount outstanding				$ 47,178	$ 0
Short-term borrowings, weighted-average interest rate				1.59%	0.00%
Senior notes due 2019 [Member]
Debt instruments [Line Items]
Debt issuance date			Mar. 15, 2011
Principal amount issued			$ 200,000
Proceeds from loan, net of offering costs			$ 196,195
Principal amount retired		$ 200,000
Stated interest rate			7.00%
Debt maturity date			Mar. 15, 2019
Credit facility [Member]
Debt instruments [Line Items]
Short-term borrowings				$ 434,000	$ 160,000

[1] We use standby letters of credit primarily to collateralize certain obligations related to our self-insured workers' compensation claims, as well as claims for environmental matters, as required by certain states. These letters of credit reduce the amount available for borrowing under our credit facility.