Revolving Line Of Credit (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Aug. 31, 2010
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity		$ 150,000,000
Revolving credit facility maturity date	Aug. 11, 2014
Outstanding borrowings	0
Unused available borrowing capacity	144,600,000
Letters of credit, outstanding	5,400,000
Performance guarantees outstanding	37,200,000
LIBOR Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	2.50%
LIBOR Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	3.75%
Bank Of America's Prime Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	1.50%
Bank Of America's Prime Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	2.75%
Letter Of Credit [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	25,000,000
Accordion Feature [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 200,000,000