Investment Securities AFS gross unreal losses (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Unrealized losses and fair value of available for sale securities , by investment category and time length
Unrealized losses on available for sale securities, less than 12 months	$ (7,615)	$ (4,832)
Unrealized losses on available for sale securities, 12 months or more	(58)	(88)
Total unrealized losses on available for sale securities	(7,673)	(4,920)
Fair value of unrealized losses on available for sale securities, less than 12 months	1,219,706	1,000,503
Fair value of unrealized losses on available for sale securities, 12 months or more	490	459
Total fair value of unrealized losses on available for sale securities	1,220,196	1,000,962
Obligations of state and political subdivisions (municipal securities) [Member]
Unrealized losses and fair value of available for sale securities , by investment category and time length
Unrealized losses on available for sale securities, less than 12 months	(36)	(42)
Unrealized losses on available for sale securities, 12 months or more	(13)	(13)
Total unrealized losses on available for sale securities	(49)	(55)
Fair value of unrealized losses on available for sale securities, less than 12 months	5,920	5,065
Fair value of unrealized losses on available for sale securities, 12 months or more	348	348
Total fair value of unrealized losses on available for sale securities	6,268	5,413
Number of available for sale securities in a continuous unrealized loss position less than twelve months	14	15
Number of available for sale securities in a continuous unrealized loss position for twelve months or more	1	1
Residential mortgage-related securities [Member]
Unrealized losses and fair value of available for sale securities , by investment category and time length
Unrealized losses on available for sale securities, less than 12 months	(6,125)	(3,727)
Unrealized losses on available for sale securities, 12 months or more	0	0
Total unrealized losses on available for sale securities	(6,125)	(3,727)
Fair value of unrealized losses on available for sale securities, less than 12 months	1,063,268	892,964
Fair value of unrealized losses on available for sale securities, 12 months or more	0	0
Total fair value of unrealized losses on available for sale securities	1,063,268	892,964
Number of available for sale securities in a continuous unrealized loss position less than twelve months	36	30
Number of available for sale securities in a continuous unrealized loss position for twelve months or more	0	0
Commercial Mortgage Backed Securities [Member]
Unrealized losses and fair value of available for sale securities , by investment category and time length
Unrealized losses on available for sale securities, less than 12 months	(1,451)	(1,063)
Unrealized losses on available for sale securities, 12 months or more	0	0
Total unrealized losses on available for sale securities	(1,451)	(1,063)
Fair value of unrealized losses on available for sale securities, less than 12 months	150,221	102,474
Fair value of unrealized losses on available for sale securities, 12 months or more	0	0
Total fair value of unrealized losses on available for sale securities	150,221	102,474
Number of available for sale securities in a continuous unrealized loss position less than twelve months	4	2
Number of available for sale securities in a continuous unrealized loss position for twelve months or more	0	0
Other Debt And Other Equity Securities [Member]
Unrealized losses and fair value of available for sale securities , by investment category and time length
Unrealized losses on available for sale securities, less than 12 months	(3)	0
Unrealized losses on available for sale securities, 12 months or more	(45)	(75)
Total unrealized losses on available for sale securities	(48)	(75)
Fair value of unrealized losses on available for sale securities, less than 12 months	297	0
Fair value of unrealized losses on available for sale securities, 12 months or more	142	111
Total fair value of unrealized losses on available for sale securities	$ 439	$ 111
Number of available for sale securities in a continuous unrealized loss position less than twelve months	1	0
Number of available for sale securities in a continuous unrealized loss position for twelve months or more	1	1

Number of available for sale securities in a continuous unrealized loss position less than twelve months

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Number of available for sale securities in a continuous unrealized loss position twelve months or longer aggregate losses.

+ References

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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