Rental Expense and Lease Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Rental Expense and Lease Commitments
Rent expense	$ 21,900,000	$ 21,600,000	$ 21,500,000
Future minimum rental commitments under non-cancelable operating leases
2013	20,498,000
2014	16,749,000
2015	13,249,000
2016	10,164,000
2017	7,149,000
Thereafter	21,577,000
Total	$ 89,386,000