Revolving Line Of Credit (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 150.0
Revolving credit facility maturity date	Aug. 11, 2014
Effective average interest rate	3.30%
Average interest rate	3.70%
Outstanding borrowings	56.0
Unused available borrowing capacity	88.1
Letters of credit, outstanding	5.9
Performance guarantees outstanding	43.9
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	25.0
Accordion Feature [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 200.0