Revolving Line Of Credit (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Aug. 11, 2010
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity	$ 150.0		$ 150.0
Revolving credit facility, maximum borrowing capacity	200.0
Revolving credit facility maturity date	Aug. 11, 2014
Effective average interest rate	3.40%	3.30%
Interest rate	2.70%
Outstanding borrowings	18.0
Unused available borrowing capacity	126.1
Letters of credit, outstanding	5.9
Performance guarantees outstanding	35.1
Interest paid	2.0	1.9
LIBOR Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	3.75%
LIBOR Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	2.50%
Bank Of America's Prime Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	2.75%
Bank Of America's Prime Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	1.50%
Letter Of Credit [Member]
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 25.0