STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 34,941	$ 46,367
Unrealized gains on securities:
Change in unrealized holding gains, net	(27,048)	(10,616)
Less: Reclassifications adjustment for gains included in net income	(5,893)	(16,541)
Change in unrealized gains on securities during the period	(32,941)	(27,157)
Income tax benefit	10,925	9,926
Other comprehensive loss	(22,016)	(17,231)
Comprehensive income	$ 12,925	$ 29,136