Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale:
Available-for-Sale: Amortized Cost	$ 3,118,846	$ 2,607,636
Available-for-Sale: Gross Unrealized Gains	75,199	21,143
Available-for-Sale: Gross Unrealized Losses	(3,732)	(9,776)
Available-for-Sale: Fair Value	3,190,313	2,619,003
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	3,198,830	3,042,294
Held-to-Maturity: Gross Unrealized Gains	90,073	29,990
Held-to-Maturity: Gross Unrealized Losses	(235)	(9,402)
Held-to-Maturity: Fair Value	3,288,668	3,062,882
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	182,605	222,365
Available-for-Sale: Gross Unrealized Gains	245	213
Available-for-Sale: Gross Unrealized Losses	(1,405)	(1,447)
Available-for-Sale: Fair Value	181,445	221,131
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	62,505	274,375
Held-to-Maturity: Gross Unrealized Gains	211	1,319
Held-to-Maturity: Gross Unrealized Losses	(2)	(31)
Held-to-Maturity: Fair Value	62,714	275,663
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Available-for-Sale: Amortized Cost	45,313	54,480
Available-for-Sale: Gross Unrealized Gains	1,491	631
Available-for-Sale: Gross Unrealized Losses	(1)	(14)
Available-for-Sale: Fair Value	46,803	55,097
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	54,001	54,811
Held-to-Maturity: Gross Unrealized Gains	1,022	1,236
Held-to-Maturity: Gross Unrealized Losses	0	0
Held-to-Maturity: Fair Value	55,023	56,047
Debt Securities Issued by Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	937	22,128
Available-for-Sale: Gross Unrealized Gains	79	19
Available-for-Sale: Gross Unrealized Losses	0	0
Available-for-Sale: Fair Value	1,016	22,147
Debt Securities Issued by Corporations
Available-for-Sale:
Available-for-Sale: Amortized Cost	221,935	335,553
Available-for-Sale: Gross Unrealized Gains	4,273	1,401
Available-for-Sale: Gross Unrealized Losses	(1,754)	(633)
Available-for-Sale: Fair Value	224,454	336,321
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	12,770	14,975
Held-to-Maturity: Gross Unrealized Gains	279	0
Held-to-Maturity: Gross Unrealized Losses	0	(138)
Held-to-Maturity: Fair Value	13,049	14,837
Residential - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	1,483,465	1,164,466
Available-for-Sale: Gross Unrealized Gains	37,344	11,627
Available-for-Sale: Gross Unrealized Losses	(307)	(3,267)
Available-for-Sale: Fair Value	1,520,502	1,172,826
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	792,360	1,067,416
Held-to-Maturity: Gross Unrealized Gains	33,324	13,247
Held-to-Maturity: Gross Unrealized Losses	(19)	(5,348)
Held-to-Maturity: Fair Value	825,665	1,075,315
Residential - U.S. Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	917,366	584,272
Available-for-Sale: Gross Unrealized Gains	19,904	4,363
Available-for-Sale: Gross Unrealized Losses	(202)	(1,874)
Available-for-Sale: Fair Value	937,068	586,761
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	2,184,472	1,546,479
Held-to-Maturity: Gross Unrealized Gains	52,270	13,871
Held-to-Maturity: Gross Unrealized Losses	(121)	(2,478)
Held-to-Maturity: Fair Value	2,236,621	1,557,872
Commercial - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	267,225	224,372
Available-for-Sale: Gross Unrealized Gains	11,863	2,889
Available-for-Sale: Gross Unrealized Losses	(63)	(2,541)
Available-for-Sale: Fair Value	279,025	224,720
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	92,722	84,238
Held-to-Maturity: Gross Unrealized Gains	2,967	317
Held-to-Maturity: Gross Unrealized Losses	(93)	(1,407)
Held-to-Maturity: Fair Value	95,596	83,148
Mortgage-Backed Securities
Available-for-Sale:
Available-for-Sale: Amortized Cost	2,668,056	1,973,110
Available-for-Sale: Gross Unrealized Gains	69,111	18,879
Available-for-Sale: Gross Unrealized Losses	(572)	(7,682)
Available-for-Sale: Fair Value	2,736,595	1,984,307
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	3,069,554	2,698,133
Held-to-Maturity: Gross Unrealized Gains	88,561	27,435
Held-to-Maturity: Gross Unrealized Losses	(233)	(9,233)
Held-to-Maturity: Fair Value	$ 3,157,882	$ 2,716,335

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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Amount, before tax, of unrealized loss 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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