Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reclassifications out of accumulated other comprehensive income
Realized losses included in net income	$ (28)		$ (28)	$ 275
Income before income taxes	23,305	20,369	64,655	57,107
Income tax expense	(8,409)	(7,364)	(23,413)	(19,820)
Net income	14,896	13,005	41,242	37,287
Unrealized gains and losses on available-for-sale securities | Amount Reclassified from Accumulated Other Comprehensive Income
Reclassifications out of accumulated other comprehensive income
Realized losses included in net income	(28)		(28)
Income before income taxes	(28)		(28)
Income tax expense	11		11
Net income	$ (17)		$ (17)