Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Securities available for sale
Available-for-sale Securities, Amortized Cost	$ 3,270,899	$ 2,933,619
Available-for-sale Securities, Gross Unrealized Gains	100,416	74,693
Available-for-sale Securities, Gross Unrealized Losses	(17,762)	(21,272)
Available-for-sale Securities, Fair Value	3,353,553	2,987,040
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	82,764	59,962
Held-to-maturity Securities, Unrecognized Holding Gain	2,348	1,832
Held-to-maturity Securities, Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	85,112	61,794
U.S. government agency debentures [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	122,711	399,122
Available-for-sale Securities, Gross Unrealized Gains	358	703
Available-for-sale Securities, Gross Unrealized Losses	0	(194)
Available-for-sale Securities, Fair Value	123,069	399,631
U.S. States and political subdivisions [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	334,916	296,327
Available-for-sale Securities, Gross Unrealized Gains	22,865	3,537
Available-for-sale Securities, Gross Unrealized Losses	(50)	(2,119)
Available-for-sale Securities, Fair Value	357,731	297,745
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	82,764	59,962
Held-to-maturity Securities, Unrecognized Holding Gain	2,348	1,832
Held-to-maturity Securities, Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	85,112	61,794
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	1,407,345	1,343,021
Available-for-sale Securities, Gross Unrealized Gains	53,129	52,230
Available-for-sale Securities, Gross Unrealized Losses	(131)	(547)
Available-for-sale Securities, Fair Value	1,460,343	1,394,704
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	1,115,832	814,774
Available-for-sale Securities, Gross Unrealized Gains	22,058	18,223
Available-for-sale Securities, Gross Unrealized Losses	(55)	(2,306)
Available-for-sale Securities, Fair Value	1,137,835	830,691
Non-agency [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	43,225	15,018
Available-for-sale Securities, Gross Unrealized Gains	82	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	43,307	15,018
Commercial Mortgage Backed Securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	127,624
Available-for-sale Securities, Gross Unrealized Gains	1,648
Available-for-sale Securities, Gross Unrealized Losses	(145)
Available-for-sale Securities, Fair Value	129,127
Corporate debt securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	115,947	61,435
Available-for-sale Securities, Gross Unrealized Gains	276	0
Available-for-sale Securities, Gross Unrealized Losses	(17,381)	(16,106)
Available-for-sale Securities, Fair Value	98,842	45,329
Total debt securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,267,600	2,929,697
Available-for-sale Securities, Gross Unrealized Gains	100,416	74,693
Available-for-sale Securities, Gross Unrealized Losses	(17,762)	(21,272)
Available-for-sale Securities, Fair Value	3,350,254	2,983,118
Marketable equity securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,299	3,922
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0
Available-for-sale Securities, Fair Value		3,922
Fair Value, Measurements, Recurring [Member]
Securities available for sale
Available-for-sale Securities, Fair Value Disclosure	3,353,553	2,987,040
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Securities available for sale
Available-for-sale Securities, Fair Value Disclosure	$ 3,299	$ 3,922

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

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