Condensed Financial Information of Registrant (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Total assets	$ 2,237,781	$ 2,091,580	$ 1,995,368
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	89	88
Additional paid-in capital	384,721	390,359
Accumulated earnings	928,720	791,209
Accumulated other comprehensive income	(6,190)	1,847
Total stockholders' equity	1,307,340	1,183,503
Parent [Member]
Assets
Investment in subsidiaries	1,208,080	1,078,606
Intercompany receivable	99,260	104,897
Total assets	1,307,340	1,183,503
Stockholders' Equity Attributable to Parent [Abstract]
Common stock	89	88
Additional paid-in capital	384,721	390,359
Accumulated earnings	928,720	791,209
Accumulated other comprehensive income	(6,190)	1,847
Total stockholders' equity	$ 1,307,340	$ 1,183,503