Debt (short-term) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended	1 Months Ended
	Mar. 31, 2015		Dec. 31, 2014	Mar. 31, 2015		Mar. 31, 2011
Debt instruments [Line Items]
Weighted-average interest rate	1.64%		1.63%
Credit facility, date of expiration	Feb. 21, 2019
Credit facility, daily average amount outstanding	$ 174,900		$ 43,675
Short-term borrowings	318,000		160,000	318,000
Interest rate, amount outstanding	1.64%		1.63%	1.64%
Credit facility commitment	350,000			350,000
Outstanding letters of credit	(12,953)	[1]		(12,953)	[1]
Credit facility, net available for borrowing at end of period	94,047			94,047
Minimum [Member]
Debt instruments [Line Items]
Credit facility, commitment fee	0.20%
Maximum [Member]
Debt instruments [Line Items]
Credit facility, commitment fee	0.40%
Short-term loan [Member]
Debt instruments [Line Items]
Debt issuance date				Mar. 05, 2015
Principal amount issued	75,000			75,000
Weighted-average interest rate	1.51%
Debt maturity date, start				Sep. 05, 2015
Principal amount over which payment is required, first payment date	50,000			50,000
Debt maturity date, end				Mar. 03, 2016
Proceeds from loan, net of offering costs				74,903
Short-term borrowings	75,000		0	75,000
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	$ 243,000		$ 160,000	$ 243,000

[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.