Debt Level 4 (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
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	18,700,000
Line of Credit Facility, Remaining Borrowing Capacity	181,300,000
Performance Bonds	21,900,000
Long-term Debt	250,048,000	234,073,000
Long-term Debt, Excluding Current Maturities	250,013,000	234,033,000
Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 28, 2016
Long-term Debt	100,000,000	84,000,000
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	150,000,000
Long-term Debt	150,000,000	150,000,000
senior notes series a [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	75,000,000
Debt Instrument, Maturity Date	Sep. 30, 2021
Debt Instrument, Interest Rate, Effective Percentage	4.01%
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Long-term Debt	48,000	73,000
Short-term Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	35,000	40,000
Senior Notes Series B [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 75,000,000
Debt Instrument, Maturity Date	Sep. 30, 2023
Debt Instrument, 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%