Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Other Comprehensive Income [Abstract]
Net income	$ 90,898	$ 81,132	$ 69,421
Other comprehensive income (loss):
Change in unrealized gains on securities	1,645	1,750	1,229
Less: reclassification adjustment for gains in net income	(30)	(9,899)	(2)
Decrease in fair value of cash flow hedge	(1,241)	0	0
Total other comprehensive income (loss)	374	(8,149)	1,227
Comprehensive income	91,272	72,983	70,648
Less: comprehensive income attributable to noncontrolling interest	(167)	0	0
Comprehensive income attributable to common stockholders	$ 91,105	$ 72,983	$ 70,648