Consolidated Statements of Comprehesive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Other Comprehensive Income [Abstract]
Net income	$ 122,063	$ 72,120	$ 204,243
Unrealized gains (losses) on investments available-for-sale:
Unrealized holding gains arising during the period	65,462	54,607	29,749
Less: Reclassification adjustments for gains included in earnings	(36,680)	(33,029)	(23,019)
Unrealized gains on investments available-for-sale, net	28,782	21,578	6,730
Defined benefit pension plans:
Prior service cost arising during the period	0	(1,304)	0
Net loss arising during the period	(646)	(12,904)	(6,806)
Less: Amortization of prior service cost and net loss included in net periodic pension cost	4,152	628	427
Defined benefit pension plans, net	(3,506)	13,580	6,379
Other comprehensive income, before tax	32,288	7,998	351
Income tax expense related to components of other comprehensive income	21,936	(8,147)	(84)
Other comprehensive income, net of tax	10,352	16,145	435
Comprehensive income	$ 132,415	$ 88,265	$ 204,678