Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt securities available for sale:
Amortized Cost	$ 5,494,622	$ 2,497,004
Gross Unrealized Gains	91,366	9,268
Gross Unrealized Losses	6,153	36,207
Available-for-sale, at fair value	5,579,835	2,470,065
Debt securities held to maturity:
Amortized Cost	144,000	148,852
Gross Unrealized Gains	6,122	1,460
Gross Unrealized Losses	194	1,045
Fair Value	149,928	149,267
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	247,666	0
Gross Unrealized Gains	2	0
Gross Unrealized Losses	521	0
Available-for-sale, at fair value	247,147	0
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	779,263	566,304
Gross Unrealized Gains	11,017	46
Gross Unrealized Losses	609	9,479
Available-for-sale, at fair value	789,671	556,871
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	4,467,243	1,930,700
Gross Unrealized Gains	80,347	9,222
Gross Unrealized Losses	4,998	26,728
Available-for-sale, at fair value	4,542,592	1,913,194
Agency mortgage-backed securities
Debt securities available for sale:
Amortized Cost	3,456,110	1,845,451
Gross Unrealized Gains	66,929	8,026
Gross Unrealized Losses	4,327	26,728
Available-for-sale, at fair value	3,518,712	1,826,749
Debt securities held to maturity:
Amortized Cost	140,324	146,052
Gross Unrealized Gains	6,122	1,460
Gross Unrealized Losses	194	1,045
Fair Value	146,252	146,467
Non-agency mortgage-backed securities
Debt securities available for sale:
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale, at fair value	0	0
Agency collateralized mortgage obligations
Debt securities available for sale:
Amortized Cost	565,959	85,245
Gross Unrealized Gains	12,484	1,196
Gross Unrealized Losses	101	0
Available-for-sale, at fair value	578,342	86,441
Non-agency collateralized mortgage obligations
Debt securities available for sale:
Amortized Cost	445,174	4
Gross Unrealized Gains	934	0
Gross Unrealized Losses	570	0
Available-for-sale, at fair value	445,538	4
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	450	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	25	0
Available-for-sale, at fair value	425	0
Debt securities held to maturity:
Amortized Cost	3,676	2,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 3,676	$ 2,800

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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