Revolving Line Of Credit (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Aug. 31, 2010
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 150.0	$ 150.0	$ 150.0
Revolving credit facility maturity date		Aug. 11, 2014
Effective average interest rate	3.40%	3.40%
Average interest rate	3.40%	3.40%
Outstanding borrowings	65.7	65.7
Unused available borrowing capacity	78.4	78.4
Letters of credit, outstanding	5.9	5.9
Performance guarantees outstanding	46.3	46.3
LIBOR Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	2.50%	2.50%
LIBOR Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Interest rate margin over reference rate	3.75%	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%	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%	2.75%
Letter Of Credit [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	25.0	25.0
Accordion Feature [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 200.0	$ 200.0