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Investment Securities - Analysis of the Contractual Maturities of Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract] |  |
Due in One Year or Less | $ 26,227 |
Due After One Year Through Five Years | 111,281 |
Due After Five Years Through Ten Years | 270,764 |
Amortized Cost, Total | 408,272 |
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract] |  |
Available-for-Sale: Amortized Cost, Total | 4,362,585 | $ 3,721,636
Due in One Year or Less | 26,290 |
Due After One Year Through Five Years | 113,889 |
Due After Five Years Through Ten Years | 271,069 |
Fair Value, Total | 411,248 |
Available-for-Sale: Fair Value | 4,353,520 | 3,791,689
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract] |  |
Due in One Year or Less | 33,254 |
Due After One Year Through Five Years | 17,290 |
Due After Five Year Through Ten Years | 123,966 |
Due After Ten Years | 11,342 |
Amortized Cost, Total | 185,852 |
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract] |  |
Due in One Year or Less | 33,379 |
Due After One Year Through Five Years | 17,285 |
Due After Five Year Through Ten Years | 124,204 |
Due After Ten Years | 11,071 |
Fair Value, Total | 185,939 |
Held-to-Maturity: Amortized Cost | 4,899,880 |
Held-to-Maturity: Fair Value | 4,895,763 | 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 | 142,664 |
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value | 144,113 |
Residential - Government Agencies |  |
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract] |  |
Available-for-sale Securities: Amortized Cost | 1,410,231 |
Available-for-Sale: Amortized Cost, Total | 1,410,231 | 1,561,603
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value | 1,417,183 |
Available-for-Sale: Fair Value | 1,417,183 | 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 | 1,877,804 |
Held-to-Maturity: Mortgage-Backed Securities: Fair Value | 1,881,962 |
Held-to-Maturity: Fair Value | 1,881,962 | 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 | 2,223,697 |
Available-for-Sale: Amortized Cost, Total | 2,223,697 | 1,497,353
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value | 2,200,052 |
Available-for-Sale: Fair Value | 2,200,052 | 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 | 2,371,155 |
Held-to-Maturity: Mortgage-Backed Securities: Fair Value | 2,369,441 |
Held-to-Maturity: Fair Value | 2,369,441 | 2,150,497
Commercial - Government Agencies |  |
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract] |  |
Available-for-sale Securities: Amortized Cost | 177,721 |
Available-for-Sale: Amortized Cost, Total | 177,721 | 243,029
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value | 180,924 |
Available-for-Sale: Fair Value | 180,924 | 253,839
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract] |  |
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost | 465,069 |
Held-to-Maturity: Mortgage-Backed Securities: Fair Value | 458,421 |
Held-to-Maturity: Fair Value | 458,421 | 195,900
Mortgage-Backed Securities |  |
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract] |  |
Available-for-sale Securities: Amortized Cost | 3,811,649 |
Available-for-Sale: Amortized Cost, Total | 3,811,649 | 3,301,985
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value | 3,798,159 |
Available-for-Sale: Fair Value | 3,798,159 | 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 | 4,714,028 |
Held-to-Maturity: Mortgage-Backed Securities: Fair Value | 4,709,824 |
Held-to-Maturity: Fair Value | $ 4,709,824 | $ 3,294,407

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