Credit Facilities Three Month Prior Year Credit Faciltiy Metrics (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Average Amount Outstanding in Canadian Dollars [Member]
Credit Facilities Summary [Line Items]
Canada Lease Warehouse Facility	$ 296,900,000	$ 41,600,000
Maximum Amount Outstanding in Canadian Dollars [Member]
Credit Facilities Summary [Line Items]
Canada Lease Warehouse Facility	368,500,000	184,600,000
Weighted Average Interest Rate [Member]
Credit Facilities Summary [Line Items]
Canada Lease Warehouse Facility	0.0270	0.0269
Syndicated warehouse facility	0.0144	0.0156
U.S. Lease Warehouse Facility		0.0160
Average Amount Outstanding [Member]
Credit Facilities Summary [Line Items]
Canada Lease Warehouse Facility	298,212,000	40,849,000
Syndicated warehouse facility	80,727,000	296,576,000
U.S. Lease Warehouse Facility		24,027,000
Maximum Amount Outstanding [Member]
Credit Facilities Summary [Line Items]
Canada Lease Warehouse Facility	370,076,000	181,314,000
Syndicated warehouse facility	621,257,000	826,859,000
U.S. Lease Warehouse Facility		$ 182,749,000