Condensed Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net Income	$ 19,792	$ 14,783	$ 41,559	$ 28,968
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustment, net of tax of $8 and $11 for the three months ended June 30, 2014 and 2013, respectively and $(7) and $11 for the six months ended June 30, 2014 and 2013, respectively	(11)	(18)	13	(18)
Unrealized gain (loss) on available-for-sale securities, net of tax of $(75) and $57 for the three months ended June 30, 2014 and 2013, respectively and $(115) and $178 for the six months ended June 30, 2014 and 2013, respectively	122	(94)	185	139
Other comprehensive income (loss)	111	(112)	198	121
Comprehensive income	19,903	14,671	41,757	29,089
Comprehensive (income) loss attributable to noncontrolling interests	(249)	602	(78)	377
Comprehensive income attributable to common stockholders	$ 19,654	$ 15,273	$ 41,679	$ 29,466