Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt securities available for sale:
Amortized Cost	$ 6,951,283	$ 6,639,277
Gross Unrealized Gains	271,732	91,083
Gross Unrealized Losses	3,642	10,359
Fair Value	7,219,373	6,720,001
Debt securities held to maturity:
Amortized Cost	130,101	139,445
Gross Unrealized Gains	11,006	5,576
Gross Unrealized Losses	0	177
Fair Value	141,107	144,844
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	209,771	235,045
Gross Unrealized Gains	504	18
Gross Unrealized Losses	156	678
Fair Value	210,119	234,385
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	810,091	852,096
Gross Unrealized Gains	30,967	12,446
Gross Unrealized Losses	2,913	687
Fair Value	838,145	863,855
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	5,930,968	5,551,685
Gross Unrealized Gains	240,261	78,619
Gross Unrealized Losses	558	8,973
Fair Value	6,170,671	5,621,331
Residential agency
Debt securities available for sale:
Amortized Cost	4,981,205	4,492,427
Gross Unrealized Gains	192,830	68,797
Gross Unrealized Losses	523	6,103
Fair Value	5,173,512	4,555,121
Debt securities held to maturity:
Amortized Cost	126,429	135,769
Gross Unrealized Gains	11,006	5,576
Gross Unrealized Losses	0	177
Fair Value	137,435	141,168
Residential non-agency
Debt securities available for sale:
Amortized Cost	265,927	374,046
Gross Unrealized Gains	4,540	1,166
Gross Unrealized Losses	35	616
Fair Value	270,432	374,596
Commercial agency
Debt securities available for sale:
Amortized Cost	644,458	645,814
Gross Unrealized Gains	40,048	8,639
Gross Unrealized Losses	0	2,049
Fair Value	684,506	652,404
Commercial non-agency
Debt securities available for sale:
Amortized Cost	39,378	39,398
Gross Unrealized Gains	2,843	17
Gross Unrealized Losses	0	205
Fair Value	42,221	39,210
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	453	451
Gross Unrealized Gains	0	0
Gross Unrealized Losses	15	21
Fair Value	438	430
Debt securities held to maturity:
Amortized Cost	3,672	3,676
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 3,672	$ 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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