Investment Securities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Held-to-maturity securities
Amortized cost	$ 2,400,000	$ 2,400,000
Estimated fair value	2,400,000	2,400,000
Carrying value	2,400,000us-gaap_HeldToMaturitySecurities	2,400,000us-gaap_HeldToMaturitySecurities
Available-for-sale securities
Amortized cost	4,936,495,000	4,914,428,000
Gross Unrealized Gains	81,300,000	68,516,000
Gross Unrealized Losses	(29,615,000)	(50,981,000)
Total investment securities	4,988,180,000us-gaap_AvailableForSaleSecurities	4,931,963,000us-gaap_AvailableForSaleSecurities
Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	4,988,180,000	4,931,963,000
Carrying value
Available-for-sale securities
Total investment securities	4,988,180,000	4,931,963,000
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,624,803,000	4,597,590,000
Gross Unrealized Gains	59,442,000	47,960,000
Gross Unrealized Losses	(23,638,000)	(45,178,000)
Residential mortgage-backed securities \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	4,660,607,000	4,600,372,000
Residential mortgage-backed securities \| Carrying value
Available-for-sale securities
Total investment securities	4,660,607,000	4,600,372,000
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Total investment securities	1,456,096	1,503,774,000
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Total investment securities	3,181,089	3,072,535,000
Mortgage-backed securities by non-government entities
Available-for-sale securities
Total investment securities	23,422	24,063,000
States and political subdivisions
Available-for-sale securities
Amortized cost	263,617,000	268,763,000
Gross Unrealized Gains	19,981,000	19,131,000
Gross Unrealized Losses	(5,844,000)	(5,618,000)
States and political subdivisions \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	277,754,000	282,276,000
States and political subdivisions \| Carrying value
Available-for-sale securities
Total investment securities	277,754,000	282,276,000
Other securities
Held-to-maturity securities
Amortized cost	2,400,000	2,400,000
Estimated fair value	2,400,000	2,400,000
Carrying value	2,400,000	2,400,000
Available-for-sale securities
Amortized cost	20,000,000	20,000,000
Other securities \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	20,000,000	20,000,000
Other securities \| Carrying value
Available-for-sale securities
Total investment securities	20,000,000	20,000,000
Other
Available-for-sale securities
Amortized cost	28,075,000	28,075,000
Gross Unrealized Gains	1,877,000	1,425,000
Gross Unrealized Losses	(133,000)	(185,000)
Other \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	29,819,000	29,315,000
Other \| Carrying value
Available-for-sale securities
Total investment securities	$ 29,819,000	$ 29,315,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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