Investment Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Available-for-Sale:
Amortized Cost	$ 3,291,685,000	$ 3,386,936,000	$ 6,457,151,000
Gross Unrealized Gains	78,333,000	68,661,000	109,238,000
Gross Unrealized Losses	(2,461,000)	(3,712,000)	(32,515,000)
Fair Value	3,367,557,000	3,451,885,000	6,533,874,000
Held-to-Maturity Securities:
Amortized Cost	3,595,065,000	3,657,796,000	127,249,000
Gross Unrealized Gains	93,977,000	99,328,000	6,779,000
Gross Unrealized Losses	(1,366,000)	(2,918,000)
Fair Value	3,687,676,000	3,754,206,000	134,028,000
Reclassification at fair value in available-for-sale investment securities to the held-to-maturity category	3,200,000,000
Unrealized after-tax gains in accumulated other comprehensive income to be amortized over the estimated remaining life of the securities	27,900,000
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Amortized Cost	855,070,000	1,220,282,000	536,770,000
Gross Unrealized Gains	14,936,000	11,204,000	19,131,000
Gross Unrealized Losses	(17,000)	(468,000)	(45,000)
Fair Value	869,989,000	1,231,018,000	555,856,000
Held-to-Maturity Securities:
Amortized Cost	190,168,000	179,474,000
Gross Unrealized Gains	5,198,000	6,704,000
Fair Value	195,366,000	186,178,000
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Amortized Cost	753,207,000	391,276,000	113,715,000
Gross Unrealized Gains	30,159,000	15,783,000	1,477,000
Gross Unrealized Losses	(955,000)		(1,583,000)
Fair Value	782,411,000	407,059,000	113,609,000
Debt Securities Issued by Corporations
Available-for-Sale:
Amortized Cost	82,450,000	97,917,000
Gross Unrealized Gains	1,984,000	607,000
Gross Unrealized Losses	0	(2,137,000)
Fair Value	84,434,000	96,387,000
Held-to-Maturity Securities:
Amortized Cost	24,000,000
Gross Unrealized Gains	4,000
Fair Value	24,004,000
Debt Securities Issued by U.S. Government-Sponsored Enterprises
Available-for-Sale:
Amortized Cost			500,000
Gross Unrealized Gains			5,000
Fair Value			505,000
Mortgage-Backed Securities
Available-for-Sale:
Amortized Cost	1,600,958,000	1,677,461,000	5,806,166,000
Gross Unrealized Gains	31,254,000	41,067,000	88,625,000
Gross Unrealized Losses	(1,489,000)	(1,107,000)	(30,887,000)
Fair Value	1,630,723,000	1,717,421,000	5,863,904,000
Held-to-Maturity Securities:
Amortized Cost	3,380,897,000	3,478,322,000
Gross Unrealized Gains	88,775,000	92,624,000
Gross Unrealized Losses	(1,366,000)	(2,918,000)
Fair Value	3,468,306,000	3,568,028,000
Mortgage-Backed Securities Issued by Government Agencies
Available-for-Sale:
Amortized Cost	1,565,724,000	1,618,913,000	5,696,907,000
Gross Unrealized Gains	29,190,000	38,066,000	84,008,000
Gross Unrealized Losses	(1,489,000)	(1,107,000)	(30,887,000)
Fair Value	1,593,425,000	1,655,872,000	5,750,028,000
Held-to-Maturity Securities:
Amortized Cost	3,349,403,000	3,429,038,000	47,368,000
Gross Unrealized Gains	86,673,000	89,801,000	2,959,000
Gross Unrealized Losses	(1,366,000)	(2,918,000)
Fair Value	3,434,710,000	3,515,921,000	50,327,000
Mortgage-Backed Securities Issued by U.S. Government-Sponsored Enterprises
Available-for-Sale:
Amortized Cost	35,234,000	58,548,000	109,259,000
Gross Unrealized Gains	2,064,000	3,001,000	4,617,000
Gross Unrealized Losses	0		0
Fair Value	37,298,000	61,549,000	113,876,000
Held-to-Maturity Securities:
Amortized Cost	31,494,000	49,284,000	79,881,000
Gross Unrealized Gains	2,102,000	2,823,000	3,820,000
Fair Value	$ 33,596,000	$ 52,107,000	$ 83,701,000