Leases (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Leases (Details)
2013 (operating leases)	$ 10,952,000	$ 10,952,000
2014 (operating leases)	7,395,000	7,395,000
2015 (operating leases)	5,919,000	5,919,000
2016 (operating leases)	5,106,000	5,106,000
2017 (operating leases)	4,490,000	4,490,000
After 2017 (operating leases)	38,988,000	38,988,000
Total minimum lease payments (operating leases)	72,850,000	72,850,000
2013 (capital leases)	76,000	76,000
2014 (capital leases)	76,000	76,000
2015 (capital leases)	76,000	76,000
2016 (capital leases)	76,000	76,000
2017 (capital leases)	76,000	76,000
After 2017 (capital leases)	151,000	151,000
Total minimum lease payments (capital leases)	531,000	531,000
Rental expense for operating leases		23,900,000	18,500,000	10,900,000
Gross sale price of corporate headquarters building, sale leaseback transaction	43,800,000
Initial lease term	15 years
Aggregate base rental payable during initial lease term	52,900,000	52,900,000
Deferred gain on sale leaseback transaction	$ 8,300,000	$ 8,300,000