Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Held-to-maturity securities
Amortized Cost	$ 2,400	$ 2,400
Estimated Fair Value	2,400	2,400
Carrying Value	2,400	2,400
Available-for-sale securities
Amortized Cost	4,290,424	4,196,034
Gross Unrealized Gains	61,814	50,165
Gross Unrealized Losses	(14,298)	(46,827)
Total investment securities	4,337,940	4,199,372
Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	4,337,940	4,199,372
Carrying Value
Available-for-sale securities
Total investment securities	4,337,940	4,199,372
Residential mortgage-backed securities
Available-for-sale securities
Amortized Cost	4,002,441	3,908,809
Gross Unrealized Gains	42,217	30,959
Gross Unrealized Losses	(14,145)	(46,557)
Residential mortgage-backed securities \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	4,030,513	3,893,211
Residential mortgage-backed securities \| Carrying Value
Available-for-sale securities
Total investment securities	4,030,513	3,893,211
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Total investment securities	1,119,022	1,147,143
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Total investment securities	2,891,276	2,724,839
Mortgage-backed securities by non-government entities
Available-for-sale securities
Total investment securities	20,215	21,229
States and political subdivisions
Available-for-sale securities
Amortized Cost	259,908	259,150
Gross Unrealized Gains	18,656	18,579
Gross Unrealized Losses	(8)	(25)
States and political subdivisions \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	278,556	277,704
States and political subdivisions \| Carrying Value
Available-for-sale securities
Total investment securities	278,556	277,704
Other securities
Held-to-maturity securities
Amortized Cost	2,400	2,400
Estimated Fair Value	2,400	2,400
Carrying Value	2,400	2,400
Other
Available-for-sale securities
Amortized Cost	28,075	28,075
Gross Unrealized Gains	941	627
Gross Unrealized Losses	(145)	(245)
Other \| Assets/Liabilities Measured at Fair Value
Available-for-sale securities
Total investment securities	28,871	28,457
Other \| Carrying Value
Available-for-sale securities
Total investment securities	$ 28,871	$ 28,457

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

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