Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
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	4,290,577	4,218,841
Gross Unrealized Gains	31,387	29,343
Gross Unrealized Losses	(41,776)	(70,835)
Total Investment Securities	4,280,188	4,177,349
Assets/liabilities measured at fair value
Available-for-sale securities
Total Investment Securities	4,280,188	4,177,349
Carrying Value
Available-for-sale securities
Total Investment Securities	4,280,188	4,177,349
Residential mortgage-backed securities
Available-for-sale securities
Amortized Cost	4,031,240	3,946,144
Gross Unrealized Gains	21,637	18,246
Gross Unrealized Losses	(41,311)	(69,920)
Residential mortgage-backed securities \| Assets/liabilities measured at fair value
Available-for-sale securities
Total Investment Securities	4,011,566	3,894,470
Residential mortgage-backed securities \| Carrying Value
Available-for-sale securities
Total Investment Securities	4,011,566	3,894,470
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Total Investment Securities	700,593	850,033
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Total Investment Securities	3,310,973	3,026,832
Mortgage-backed securities by non-government entities
Available-for-sale securities
Total Investment Securities		17,605
Obligations of states and political subdivisions
Available-for-sale securities
Amortized Cost	231,262	244,622
Gross Unrealized Gains	9,393	10,783
Gross Unrealized Losses	(109)	(433)
Obligations of states and political subdivisions \| Assets/liabilities measured at fair value
Available-for-sale securities
Total Investment Securities	240,546	254,972
Obligations of states and political subdivisions \| Carrying Value
Available-for-sale securities
Total Investment Securities	240,546	254,972
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
Equity securities
Available-for-sale securities
Amortized Cost	28,075	28,075
Gross Unrealized Gains	357	314
Gross Unrealized Losses	(356)	(482)
Equity securities \| Assets/liabilities measured at fair value
Available-for-sale securities
Total Investment Securities	28,076	27,907
Equity securities \| Carrying Value
Available-for-sale securities
Total Investment Securities	$ 28,076	$ 27,907

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

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