Condensed Financial Information of Registrant (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Total assets	$ 2,091,580	$ 1,995,368	$ 1,766,422
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	88	87
Additional paid-in capital	390,359	361,715
Accumulated earnings	791,209	745,658
Accumulated other comprehensive income	1,847	418
Total stockholders' equity	1,183,503	1,107,878
Parent [Member]
Assets
Investment in subsidiaries	1,078,606	1,031,626
Intercompany receivable	104,897	76,252
Total assets	1,183,503	1,107,878
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	88	87
Additional paid-in capital	390,359	361,715
Accumulated earnings	791,209	745,658
Accumulated other comprehensive income	1,847	418
Total stockholders' equity	$ 1,183,503	$ 1,107,878