Results of Operations and Long-Lived Assets by Geography (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
Results of Operations and Long-Lived Assets by Country [Line Items]
Total revenues	$ 304,501	$ 299,088	$ 300,525	$ 286,601	$ 275,855	$ 275,791	$ 275,095	$ 276,352	$ 1,190,715	$ 1,103,093	$ 1,361,863
Income before income taxes	60,946	70,556	71,921	63,465	56,681	55,507	58,465	73,256	266,888	243,909	361,469
Long-Lived Assets	83,174				84,866				83,174	84,866
UNITED STATES
Results of Operations and Long-Lived Assets by Country [Line Items]
Total revenues									971,615	895,067	1,136,414
Income before income taxes									202,391	181,151	282,492
Long-Lived Assets	82,724				84,550				82,724	84,550
CANADA
Results of Operations and Long-Lived Assets by Country [Line Items]
Total revenues									219,100	208,026	225,449
Income before income taxes									64,497	62,758	78,977
Long-Lived Assets	$ 450				$ 316				$ 450	$ 316