Debt and Capital Lease Obligations (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total debt	$ 421,622	$ 356,000
Less - current maturities of long-term debt and capital lease obligations	114	0
Line of credit facility, current borrowing capacity	400,000	350,000
Line of Credit Facility, Extension Capacity	50,000
Line of credit facility, maximum borrowing capacity	450,000
Performance bonds under credit facility	26,100
Line of credit facility, remaining borrowing capacity	103,900
Performance bonds	12,500
Long-term Debt, Excluding Current Maturities	421,508	356,000
Senior Notes [Member]
Debt Instrument [Line Items]
Total debt	150,000	150,000
Face amount	150,000
Line of Credit [Member]
Debt Instrument [Line Items]
Total debt	$ 270,000	$ 206,000
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
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