Investment Securities - Analysis of the Contractual Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 61,913
Due After One Year Through Five Years	59,080
Due After Five Years Through Ten Years	148,282
Amortized Cost, Total	269,275
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-Sale: Amortized Cost	3,118,846
Due in One Year or Less	61,802
Due After One Year Through Five Years	60,281
Due After Five Years Through Ten Years	151,301
Fair Value, Total	273,384
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	2,736,595
Available-for-Sale: Fair Value	3,190,313
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	80,075
Due After One Year Through Five Years	49,201
Amortized Cost, Total	129,276
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	80,356
Due After One Year Through Five Years	50,430
Fair Value, Total	130,786
Held-to-Maturity: Amortized Cost	3,198,830
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	3,157,882
Held-to-Maturity: Fair Value	3,288,668	$ 3,062,882
Debt Securities Issued by Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	181,515
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	180,334
Residential - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	1,483,465
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	1,520,502
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	792,360
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	825,665
Held-to-Maturity: Fair Value	825,665	1,075,315
Residential - U.S. Government-Sponsored Enterprises
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	917,366
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	937,068
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	2,184,472
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	2,236,621
Held-to-Maturity: Fair Value	2,236,621	1,557,872
Commercial - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	267,225
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	279,025
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	92,722
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	95,596
Held-to-Maturity: Fair Value	95,596	83,148
Mortgage-Backed Securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	2,668,056
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	3,069,554
Held-to-Maturity: Fair Value	$ 3,157,882	$ 2,716,335