Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt securities available for sale:
Amortized Cost	$ 8,396,973	$ 8,041,173
Gross Unrealized Gains	150,002	247,839
Gross Unrealized Losses	143,187	4,289
Fair Value	8,403,788	8,284,723
Debt securities held to maturity:
Amortized Cost	209,778	184,359
Gross Unrealized Gains	6,615	9,267
Gross Unrealized Losses	3,982	72
Fair Value	212,411	193,554
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	824,061	827,191
Gross Unrealized Gains	30,772	44,077
Gross Unrealized Losses	8,041	1,087
Fair Value	846,792	870,181
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	186,541	196,560
Gross Unrealized Gains	506	153
Gross Unrealized Losses	649	813
Fair Value	186,398	195,900
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	7,385,917	7,016,969
Gross Unrealized Gains	118,713	203,561
Gross Unrealized Losses	134,497	2,389
Fair Value	7,370,133	7,218,141
Residential agency
Debt securities available for sale:
Amortized Cost	6,545,644	6,151,511
Gross Unrealized Gains	93,014	157,955
Gross Unrealized Losses	123,937	1,388
Fair Value	6,514,721	6,308,078
Debt securities held to maturity:
Amortized Cost	206,151	180,946
Gross Unrealized Gains	6,615	9,267
Gross Unrealized Losses	3,982	72
Fair Value	208,784	190,141
Residential non-agency
Debt securities available for sale:
Amortized Cost	119,284	156,865
Gross Unrealized Gains	1,882	2,819
Gross Unrealized Losses	3	2
Fair Value	121,163	159,682
Commercial agency
Debt securities available for sale:
Amortized Cost	681,641	669,235
Gross Unrealized Gains	23,146	39,715
Gross Unrealized Losses	10,557	999
Fair Value	694,230	707,951
Commercial non-agency
Debt securities available for sale:
Amortized Cost	39,348	39,358
Gross Unrealized Gains	671	3,072
Gross Unrealized Losses	0	0
Fair Value	40,019	42,430
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	454	453
Gross Unrealized Gains	11	48
Gross Unrealized Losses	0	0
Fair Value	465	501
Debt securities held to maturity:
Amortized Cost	3,627	3,413
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 3,627	$ 3,413

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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