Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt securities available for sale:
Amortized Cost	$ 6,639,277	$ 2,497,004
Gross Unrealized Gains	91,083	9,268
Gross Unrealized Losses	10,359	36,207
Fair Value	6,720,001	2,470,065
Debt securities held to maturity:
Amortized Cost	139,445	148,852
Gross Unrealized Gains	5,576	1,460
Gross Unrealized Losses	177	1,045
Fair Value	144,844	149,267
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	235,045
Gross Unrealized Gains	18
Gross Unrealized Losses	678
Fair Value	234,385
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	852,096	566,304
Gross Unrealized Gains	12,446	46
Gross Unrealized Losses	687	9,479
Fair Value	863,855	556,871
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	5,551,685	1,930,700
Gross Unrealized Gains	78,619	9,222
Gross Unrealized Losses	8,973	26,728
Fair Value	5,621,331	1,913,194
Agency mortgage-backed securities
Debt securities available for sale:
Amortized Cost	4,492,427	1,845,451
Gross Unrealized Gains	68,797	8,026
Gross Unrealized Losses	6,103	26,728
Fair Value	4,555,121	1,826,749
Debt securities held to maturity:
Amortized Cost	135,769	146,052
Gross Unrealized Gains	5,576	1,460
Gross Unrealized Losses	177	1,045
Fair Value	141,168	146,467
Non-agency mortgage-backed securities
Debt securities available for sale:
Amortized Cost	374,046	4
Gross Unrealized Gains	1,166	0
Gross Unrealized Losses	616	0
Fair Value	374,596	4
Agency collateralized mortgage obligations
Debt securities available for sale:
Amortized Cost	645,814	85,245
Gross Unrealized Gains	8,639	1,196
Gross Unrealized Losses	2,049	0
Fair Value	652,404	86,441
Non-agency collateralized mortgage obligations
Debt securities available for sale:
Amortized Cost	39,398
Gross Unrealized Gains	17
Gross Unrealized Losses	205
Fair Value	39,210
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	451
Gross Unrealized Gains	0
Gross Unrealized Losses	21
Fair Value	430
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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