Investment Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Available-for-Sale:
Amortized Cost	$ 3,386,936,000	$ 6,457,151,000	$ 5,289,757,000
Gross Unrealized Gains	68,661,000	109,238,000	62,856,000
Gross Unrealized Losses	(3,712,000)	(32,515,000)	(21,779,000)
Fair Value	3,451,885,000	6,533,874,000	5,330,834,000
Held-to-Maturity:
Amortized Cost	3,657,796,000	127,249,000	181,018,000
Gross Unrealized Gains	99,328,000	6,779,000	5,650,000
Gross Unrealized Losses	(2,918,000)
Fair Value	3,754,206,000	134,028,000	186,668,000
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	1,220,282,000	536,770,000	711,223,000
Gross Unrealized Gains	11,204,000	19,131,000	11,248,000
Gross Unrealized Losses	(468,000)	(45,000)	(1,679,000)
Fair Value	1,231,018,000	555,856,000	720,792,000
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Amortized Cost	391,276,000	113,715,000	52,742,000
Gross Unrealized Gains	15,783,000	1,477,000	1,391,000
Gross Unrealized Losses		(1,583,000)	(17,000)
Fair Value	407,059,000	113,609,000	54,116,000
Held-to-Maturity:
Amortized Cost	179,474,000
Gross Unrealized Gains	6,704,000
Fair Value	186,178,000
Debt Securities Issued by Corporations
Available-for-Sale:
Amortized Cost	97,917,000
Gross Unrealized Gains	607,000
Gross Unrealized Losses	(2,137,000)
Fair Value	96,387,000
Debt Securities Issued by U.S. Government-Sponsored Enterprises
Available-for-Sale:
Amortized Cost		500,000	751,000
Gross Unrealized Gains		5,000	41,000
Fair Value		505,000	792,000
Mortgage-Backed Securities
Available-for-Sale:
Amortized Cost	1,677,461,000	5,806,166,000	4,525,041,000
Gross Unrealized Gains	41,067,000	88,625,000	50,176,000
Gross Unrealized Losses	(1,107,000)	(30,887,000)	(20,083,000)
Fair Value	1,717,421,000	5,863,904,000	4,555,134,000
Held-to-Maturity:
Amortized Cost	3,478,322,000
Gross Unrealized Gains	92,624,000
Gross Unrealized Losses	(2,918,000)
Fair Value	3,568,028,000
Mortgage-Backed Securities Issued by Government Agencies
Available-for-Sale:
Amortized Cost	1,618,913,000	5,696,907,000	4,015,816,000
Gross Unrealized Gains	38,066,000	84,008,000	26,900,000
Gross Unrealized Losses	(1,107,000)	(30,887,000)	(20,029,000)
Fair Value	1,655,872,000	5,750,028,000	4,022,687,000
Held-to-Maturity:
Amortized Cost	3,429,038,000	47,368,000	59,542,000
Gross Unrealized Gains	89,801,000	2,959,000	1,879,000
Gross Unrealized Losses	(2,918,000)
Fair Value	3,515,921,000	50,327,000	61,421,000
Mortgage-Backed Securities Issued by U.S. Government-Sponsored Enterprises
Available-for-Sale:
Amortized Cost	58,548,000	109,259,000	509,225,000
Gross Unrealized Gains	3,001,000	4,617,000	23,276,000
Gross Unrealized Losses			(54,000)
Fair Value	61,549,000	113,876,000	532,447,000
Held-to-Maturity:
Amortized Cost	49,284,000	79,881,000	121,476,000
Gross Unrealized Gains	2,823,000	3,820,000	3,771,000
Fair Value	$ 52,107,000	$ 83,701,000	$ 125,247,000