Condensed Financial Information of Registrant (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Total assets	$ 1,995,368	$ 1,766,422	$ 1,627,404
Stockholders' equity:
Common stock	87	86
Additional paid-in capital	361,715	347,848
Accumulated earnings	745,658	650,253
Accumulated other comprehensive income	418	737
Total stockholders' equity	1,107,878	998,924
Parent Company [Member]
Assets
Investment in subsidiaries	1,031,626	936,540
Intercompany receivable	76,252	62,384
Total assets	1,107,878	998,924
Stockholders' equity:
Common stock	87	86
Additional paid-in capital	361,715	347,848
Accumulated earnings	745,658	650,253
Accumulated other comprehensive income	418	737
Total stockholders' equity	$ 1,107,878	$ 998,924