Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt securities available for sale:
Amortized Cost	$ 8,041,173	$ 6,639,277
Gross Unrealized Gains	247,839	91,083
Gross Unrealized Losses	4,289	10,359
Fair Value	8,284,723	6,720,001
Debt securities held to maturity:
Amortized Cost	184,359	139,445
Gross Unrealized Gains	9,267	5,576
Gross Unrealized Losses	72	177
Fair Value	193,554	144,844
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	827,191	852,096
Gross Unrealized Gains	44,077	12,446
Gross Unrealized Losses	1,087	687
Fair Value	870,181	863,855
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	196,560	235,045
Gross Unrealized Gains	153	18
Gross Unrealized Losses	813	678
Fair Value	195,900	234,385
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	7,016,969	5,551,685
Gross Unrealized Gains	203,561	78,619
Gross Unrealized Losses	2,389	8,973
Fair Value	7,218,141	5,621,331
Residential agency
Debt securities available for sale:
Amortized Cost	6,151,511	4,492,427
Gross Unrealized Gains	157,955	68,797
Gross Unrealized Losses	1,388	6,103
Fair Value	6,308,078	4,555,121
Debt securities held to maturity:
Amortized Cost	180,946	135,769
Gross Unrealized Gains	9,267	5,576
Gross Unrealized Losses	72	177
Fair Value	190,141	141,168
Residential non-agency
Debt securities available for sale:
Amortized Cost	156,865	374,046
Gross Unrealized Gains	2,819	1,166
Gross Unrealized Losses	2	616
Fair Value	159,682	374,596
Commercial agency
Debt securities available for sale:
Amortized Cost	669,235	645,814
Gross Unrealized Gains	39,715	8,639
Gross Unrealized Losses	999	2,049
Fair Value	707,951	652,404
Commercial non-agency
Debt securities available for sale:
Amortized Cost	39,358	39,398
Gross Unrealized Gains	3,072	17
Gross Unrealized Losses	0	205
Fair Value	42,430	39,210
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	453	451
Gross Unrealized Gains	48	0
Gross Unrealized Losses	0	21
Fair Value	501	430
Debt securities held to maturity:
Amortized Cost	3,413	3,676
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 3,413	$ 3,676

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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