Debt and Capital Lease Obligations (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total debt	$ 333,011,000	$ 267,028,000
Less - current maturities of long-term debt and capital lease obligations	11,000	26,000
Long-term debt and capital lease obligations, net	333,000,000	267,002,000
Line of credit facility, current borrowing capacity	300,000,000
Line of credit facility, maximum borrowing capacity	350,000,000
Performance bonds under credit facility	21,900,000
Line of credit facility, remaining borrowing capacity	95,100,000
Performance bonds	24,400,000
Senior Notes [Member]
Debt Instrument [Line Items]
Total debt	150,000,000	150,000,000
Face amount	150,000,000
Line of Credit [Member]
Debt Instrument [Line Items]
Total debt	183,000,000	117,000,000
Maturity date	Sep. 28, 2016
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Total debt	11,000	28,000
Senior Notes Series A [Member]
Debt Instrument [Line Items]
Face amount	75,000,000
Maturity date	Sep. 30, 2021
Interest rate, effective percentage	4.01%
Senior Notes Series B [Member]
Debt Instrument [Line Items]
Face amount	$ 75,000,000
Maturity date	Sep. 30, 2023
Interest rate, effective percentage	4.11%
Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.50%
Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.25%