Debt and Capital Lease Obligations (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 400,000
Line of Credit Facility, Extension Capacity	50,000
Line of credit facility, maximum borrowing capacity	450,000
Performance bonds under credit facility	25,700
Line of credit facility, remaining borrowing capacity	316,300
Performance bonds	9,400
Long-term Debt, Excluding Current Maturities	206,363	$ 430,987
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt	150,000	150,000
Face amount	150,000
Line of Credit [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 58,000	283,000
Maturity date	Aug. 29, 2019
Senior Notes Series A [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.01%
Face amount	$ 75,000
Maturity date	Sep. 30, 2021
Senior Notes Series B [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.11%
Face amount	$ 75,000
Maturity date	Sep. 30, 2023
Short-term Debt [Member]
Debt Instrument [Line Items]
Debt, Current	$ 18	2,244
Total debt [Member]
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount	208,018	435,244
Debt issuance costs [Domain]
Debt Instrument [Line Items]
Debt acquisition costs	$ (1,637)	$ (2,013)
Minimum [Member]
Debt Instrument [Line Items]
Basis Spread on Variable Rate	1.25%
Maximum [Member]
Debt Instrument [Line Items]
Basis Spread on Variable Rate	2.00%
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount	$ 50,000