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Commitments and Contingencies (Details) (USD $)	12 Months Ended
Commitments and Contingencies (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commitments and Contingencies
Rent expense	$ 22,200,000	$ 22,900,000	$ 21,700,000
2013	12,930,000
2014	9,067,000
2015	7,901,000
2016	6,682,000
2017	6,203,000
Thereafter	22,922,000
Total future minimum rental payments	65,705,000
Schedule Of Guarantees
Maximum potential undiscounted payments	343,544,000
Performance guarantees through letters of credit
Schedule Of Guarantees
Term	2013-2017
Maximum potential undiscounted payments	246,017,000
Standby letters of credit
Schedule Of Guarantees
Term	2013
Maximum potential undiscounted payments	13,466,000
Commercial letters of credit
Schedule Of Guarantees
Term	2013
Maximum potential undiscounted payments	1,736,000
Bid bonds and performance bonds
Schedule Of Guarantees
Term	2013-2018
Maximum potential undiscounted payments	$ 82,325,000