Consolidated Statement Of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Consolidated Statement Of Comprehensive Income [Abstract]
Net income attributable to common shareholders	$ 41,445		$ 37,590		$ 79,349		$ 67,941
Other comprehensive income, net of tax:
Unrealized holding gain (loss) on investments	(21)	[1],[2]	92	[1],[2]	97	[1],[2]	96	[1],[2]
Reclassification adjustment for gain reported in net income			(71)	[3],[4]	(166)	[3],[4]	(73)	[3],[4]
Comprehensive income	$ 41,424		$ 37,611		$ 79,280		$ 67,964

[1] amounts are net of tax of $51 and $51 for the six months ended June 30, 2012 and 2011, respectively

[2] amounts are net of tax of $12 and $49 for the three months ended June 30, 2012 and 2011, respectively

[3] amounts are net of tax of $90 and $39 for the six months ended June 30, 2012 and 2011, respectively

[4] amount is net of tax of $38 for the three months ended June 30, 2011