Consolidated Statements of Comprehensive Income (Loss) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 28,478	$ 27,206	$ 52,423	$ 48,929
Change in securities available-for-sale:
Unrealized holding gains (losses) for the period	(73,602)	11,221	(87,385)	17,461
Reclassification adjustment for securities gains realized in income	(1,789)	(6,992)	(2,808)	(7,611)
Other-than-temporary-impairment on available-for-sale securities associated with credit loss realized in income		780		876
Income tax effect	28,957	(1,859)	34,366	(4,175)
Unrealized gains on available-for-sale securities	(46,434)	3,150	(55,827)	6,551
Change in securities held-to-maturity:
Amortization of fair value for securities held-to-maturity previously recognized into accumulated other comprehensive income	(177)	(231)	(354)	(461)
Income tax effect	70	92	141	184
Changes from securities held-to-maturity	(107)	(139)	(213)	(277)
Cash flow hedges:
Net unrealized derivative gains (losses) on cash flow hedges	874		874	(240)
Income tax effect	(349)		(349)	96
Changes from cash flow hedges	525		525	(144)
Defined benefit pension plans:
Amortization of net loss recognized in income	842	1,007	1,702	2,014
Income tax effect	(665)	(403)	(1,009)	(806)
Changes from defined benefit pension plans	177	604	693	1,208
Other comprehensive income (loss), net of tax	(45,839)	3,615	(54,822)	7,338
Comprehensive income (loss)	$ (17,361)	$ 30,821	$ (2,399)	$ 56,267