Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Available-for-Sale:
Available-for-Sale: Amortized Cost	$ 3,721,636	$ 2,607,636	$ 2,022,158
Available-for-Sale: Gross Unrealized Gains	72,428	21,143	9,225
Available-for-Sale: Gross Unrealized Losses	(2,375)	(9,776)	(23,441)
Available-for-Sale: Fair Value	3,791,689	2,619,003	2,007,942
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	3,262,727	3,042,294	3,482,092
Held-to-Maturity: Gross Unrealized Gains	86,494	29,990	11,959
Held-to-Maturity: Gross Unrealized Losses	(528)	(9,402)	(80,057)
Held-to-Maturity: Fair Value	3,348,693	3,062,882	3,413,994
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	174,409	222,365	394,485
Available-for-Sale: Gross Unrealized Gains	427	213	493
Available-for-Sale: Gross Unrealized Losses	(591)	(1,447)	(2,577)
Available-for-Sale: Fair Value	174,245	221,131	392,401
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	7,500	274,375	353,122
Held-to-Maturity: Gross Unrealized Gains	8	1,319	186
Held-to-Maturity: Gross Unrealized Losses	(8)	(31)	(1,093)
Held-to-Maturity: Fair Value	7,500	275,663	352,215
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Available-for-Sale: Amortized Cost	23,540	54,480	559,800
Available-for-Sale: Gross Unrealized Gains	1,301	631	5,227
Available-for-Sale: Gross Unrealized Losses	(1)	(14)	(1,031)
Available-for-Sale: Fair Value	24,840	55,097	563,996
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	33,763	54,811	234,602
Held-to-Maturity: Gross Unrealized Gains	741	1,236	6,150
Held-to-Maturity: Fair Value	34,504	56,047	240,752
Debt Securities Issued by Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	985	22,128	56
Available-for-Sale: Gross Unrealized Gains	77	19	0
Available-for-Sale: Gross Unrealized Losses	0	0	0
Available-for-Sale: Fair Value	1,062	22,147	56
Debt Securities Issued by Corporations
Available-for-Sale:
Available-for-Sale: Amortized Cost	220,717	335,553	224,997
Available-for-Sale: Gross Unrealized Gains	4,844	1,401	0
Available-for-Sale: Gross Unrealized Losses	(956)	(633)	(1,857)
Available-for-Sale: Fair Value	224,605	336,321	223,140
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	12,031	14,975	97,266
Held-to-Maturity: Gross Unrealized Gains	251
Held-to-Maturity: Gross Unrealized Losses		(138)	(1,755)
Held-to-Maturity: Fair Value	12,282	14,837	95,511
Residential - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	1,561,603	1,164,466	189,645
Available-for-Sale: Gross Unrealized Gains	33,657	11,627	1,726
Available-for-Sale: Gross Unrealized Losses	(445)	(3,267)	(929)
Available-for-Sale: Fair Value	1,594,815	1,172,826	190,442
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	917,459	1,067,416	1,861,874
Held-to-Maturity: Gross Unrealized Gains	30,580	13,247	3,886
Held-to-Maturity: Gross Unrealized Losses	(29)	(5,348)	(51,773)
Held-to-Maturity: Fair Value	948,010	1,075,315	1,813,987
Residential - U.S. Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	1,497,353	584,272	589,311
Available-for-Sale: Gross Unrealized Gains	21,254	4,363	1,779
Available-for-Sale: Gross Unrealized Losses	(324)	(1,874)	(12,563)
Available-for-Sale: Fair Value	1,518,283	586,761	578,527
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	2,099,053	1,546,479	758,835
Held-to-Maturity: Gross Unrealized Gains	51,735	13,871	1,590
Held-to-Maturity: Gross Unrealized Losses	(291)	(2,478)	(20,259)
Held-to-Maturity: Fair Value	2,150,497	1,557,872	740,166
Commercial - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	243,029	224,372	63,864
Available-for-Sale: Gross Unrealized Gains	10,868	2,889	0
Available-for-Sale: Gross Unrealized Losses	(58)	(2,541)	(4,484)
Available-for-Sale: Fair Value	253,839	224,720	59,380
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	192,921	84,238	176,393
Held-to-Maturity: Gross Unrealized Gains	3,179	317	147
Held-to-Maturity: Gross Unrealized Losses	(200)	(1,407)	(5,177)
Held-to-Maturity: Fair Value	195,900	83,148	171,363
Mortgage-Backed Securities
Available-for-Sale:
Available-for-Sale: Amortized Cost	3,301,985	1,973,110	842,820
Available-for-Sale: Gross Unrealized Gains	65,779	18,879	3,505
Available-for-Sale: Gross Unrealized Losses	(827)	(7,682)	(17,976)
Available-for-Sale: Fair Value	3,366,937	1,984,307	828,349
Held-to-Maturity:
Held-to-Maturity: Amortized Cost	3,209,433	2,698,133	2,797,102
Held-to-Maturity: Gross Unrealized Gains	85,494	27,435	5,623
Held-to-Maturity: Gross Unrealized Losses	(520)	(9,233)	(77,209)
Held-to-Maturity: Fair Value	$ 3,294,407	$ 2,716,335	$ 2,725,516

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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Amortized cost of 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 of 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 of investment in debt security measured at amortized cost (held-to-maturity).

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