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Investment Securities - Analysis of the Contractual Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 51,611
Due After One Year Through Five Years	111,529
Due After Five Years Through Ten Years	298,653
Amortized Cost, Total	461,793
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-Sale: Amortized Cost, Total	4,521,660	$ 3,721,636
Due in One Year or Less	51,642
Due After One Year Through Five Years	114,504
Due After Five Years Through Ten Years	301,024
Fair Value, Total	467,170
Available-for-Sale: Fair Value	4,522,941	3,791,689
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	33,425
Due After One Year Through Five Years	18,040
Due After Five Year Through Ten Years	123,935
Amortized Cost, Total	175,400
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	33,769
Due After One Year Through Five Years	18,105
Due After Five Year Through Ten Years	124,850
Fair Value, Total	176,724
Held-to-Maturity: Amortized Cost	3,947,613
Held-to-Maturity: Fair Value	3,965,117	3,348,693
Debt Securities Issued by Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	153,469
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	154,990
Residential - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	1,538,897
Available-for-Sale: Amortized Cost, Total	1,538,897	1,561,603
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	1,544,673
Available-for-Sale: Fair Value	1,544,673	1,594,815
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	1,536,020
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	1,541,977
Held-to-Maturity: Fair Value	1,541,977	948,010
Residential - U.S. Government-Sponsored Enterprises
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	2,170,640
Available-for-Sale: Amortized Cost, Total	2,170,640	1,497,353
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	2,154,535
Available-for-Sale: Fair Value	2,154,535	1,518,283
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	1,929,372
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	1,941,887
Held-to-Maturity: Fair Value	1,941,887	2,150,497
Commercial - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	196,861
Available-for-Sale: Amortized Cost, Total	196,861	243,029
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	201,573
Available-for-Sale: Fair Value	201,573	253,839
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	306,821
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	304,529
Held-to-Maturity: Fair Value	304,529	195,900
Mortgage-Backed Securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	3,906,398
Available-for-Sale: Amortized Cost, Total	3,906,398	3,301,985
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	3,900,781
Available-for-Sale: Fair Value	3,900,781	3,366,937
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	3,772,213
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	3,788,393
Held-to-Maturity: Fair Value	$ 3,788,393	$ 3,294,407

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