Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 66,533	$ 64,943	$ 69,973	$ 68,594
Other comprehensive income (loss):
Foreign currency translation adjustments	(1,159)	(177)	(232)	(331)
Change in unrealized gains and losses on interest rate swaps, net of tax	994	(1,279)	1,204	(1,335)
Total other comprehensive income (loss)	(165)	(1,456)	972	(1,666)
Comprehensive income	$ 66,368	$ 63,487	$ 70,945	$ 66,928