Investment Securities - Analysis of the Contractual Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 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	$ 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