Segment and Geographic Area Information Operating Revenues by Geographic Area (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues	$ 227,316	$ 222,745	$ 217,419	$ 207,436	$ 210,386	$ 217,210	$ 209,589	$ 191,911	$ 874,916	$ 829,096	$ 630,195
Percent of Total Operating Revenue									100.00%	100.00%	100.00%
United States [Member]
Segment Reporting Information [Line Items]
Revenues									489,157	478,511	434,132
Percent of Total Operating Revenue									55.90%	57.70%	68.90%
Australia [Member]
Segment Reporting Information [Line Items]
Revenues									289,023	271,475	135,408
Percent of Total Operating Revenue									33.00%	32.70%	21.50%
Canada [Member]
Segment Reporting Information [Line Items]
Revenues									81,948	64,155	49,580
Percent of Total Operating Revenue									9.40%	7.80%	7.90%
Europe [Member]
Segment Reporting Information [Line Items]
Revenues									14,788	14,955	11,075
Percent of Total Operating Revenue									1.70%	1.80%	1.70%
Total Non-United States
Segment Reporting Information [Line Items]
Revenues									$ 385,759	$ 350,585	$ 196,063
Percent of Total Operating Revenue									44.10%	42.30%	31.10%