Segment and Geographic Area Information (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Revenues	$ 210,386	$ 217,210	$ 209,589	$ 191,911	$ 169,671	$ 156,492	$ 158,453	$ 145,579	$ 829,096	$ 630,195	$ 544,866
Percent of Total Operating Revenue									100.00%	100.00%	100.00%
United States [Member]
Segment Reporting Information [Line Items]
Revenues									478,511	434,132	403,239
Percent of Total Operating Revenue									57.70%	68.90%	74.00%
Australia [Member]
Segment Reporting Information [Line Items]
Revenues									271,475	135,408	93,420
Percent of Total Operating Revenue									32.70%	21.50%	17.10%
Canada [Member]
Segment Reporting Information [Line Items]
Revenues									64,155	49,580	38,061
Percent of Total Operating Revenue									7.80%	7.90%	7.00%
Europe [Member]
Segment Reporting Information [Line Items]
Revenues									14,955	11,075	10,146
Percent of Total Operating Revenue									1.80%	1.70%	1.90%
Total Non-United States
Segment Reporting Information [Line Items]
Revenues									$ 350,585	$ 196,063	$ 141,627
Percent of Total Operating Revenue									42.30%	31.10%	26.00%