Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 17,620	$ 20,387
Change in unrealized gain on investments:
Unrealized gain arising during the period	2,306	275
Call and repayment losses charged to investment income	15	15
Reclassification adjustment for realized gains	(4)	(20)
Net change in unrealized gain	2,317	270
Deferred income taxes on above change	(893)	(105)
Total other comprehensive income, net of income taxes	1,424	165
Comprehensive income	$ 19,044	$ 20,552