Investment Securities - Analysis of the Contractual Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 52,461
Due After One Year Through Five Years	46,727
Due After Five Years Through Ten Years	146,958
Amortized Cost, Total	246,146
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-Sale: Amortized Cost, Total	3,721,636	$ 2,607,636	$ 2,022,158
Due in One Year or Less	52,378
Due After One Year Through Five Years	47,767
Due After Five Years Through Ten Years	151,282
Fair Value, Total	251,427
Available-for-Sale: Fair Value	3,791,689	2,619,003	2,007,942
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	33,763
Due After One Year Through Five Years	19,531
Amortized Cost, Total	53,294
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	34,504
Due After One Year Through Five Years	19,782
Fair Value, Total	54,286
Held-to-Maturity: Amortized Cost	3,262,727
Held-to-Maturity: Fair Value	3,348,693	3,062,882	3,413,994
Debt Securities Issued by Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	173,505
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	173,325
Residential - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	1,561,603
Available-for-Sale: Amortized Cost, Total	1,561,603	1,164,466	189,645
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	1,594,815
Available-for-Sale: Fair Value	1,594,815	1,172,826	190,442
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	917,459
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	948,010
Held-to-Maturity: Fair Value	948,010	1,075,315	1,813,987
Residential - U.S. Government-Sponsored Enterprises
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	1,497,353
Available-for-Sale: Amortized Cost, Total	1,497,353	584,272	589,311
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	1,518,283
Available-for-Sale: Fair Value	1,518,283	586,761	578,527
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	2,099,053
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	2,150,497
Held-to-Maturity: Fair Value	2,150,497	1,557,872	740,166
Commercial - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	243,029
Available-for-Sale: Amortized Cost, Total	243,029	224,372	63,864
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	253,839
Available-for-Sale: Fair Value	253,839	224,720	59,380
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	192,921
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	195,900
Held-to-Maturity: Fair Value	195,900	83,148	171,363
Mortgage-Backed Securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities: Amortized Cost	3,301,985
Available-for-Sale: Amortized Cost, Total	3,301,985	1,973,110	842,820
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	3,366,937
Available-for-Sale: Fair Value	3,366,937	1,984,307	828,349
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	3,209,433
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	3,294,407
Held-to-Maturity: Fair Value	$ 3,294,407	$ 2,716,335	$ 2,725,516