Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 45,912	$ 66,124
Due in 1-5 years	163,346	191,364
Due in 5-10 years	681,135	547,813
Due after 10 years	7,150,780	5,833,976
Amortized Cost	8,041,173	6,639,277
Fair Value
Due in one year or less	46,163	66,112
Due in 1-5 years	168,125	192,065
Due in 5-10 years	720,239	555,523
Due after 10 years	7,350,196	5,906,301
Total investment securities available-for-sale	8,284,723	6,720,001
Amortized Cost
Due in one year or less	400	0
Due in 1-5 years	2,150	3,550
Due in 5-10 years	46	58
Due after 10 years	181,763	135,837
Amortized Cost	184,359	139,445
Fair Value
Due in one year or less	400	0
Due in 1-5 years	2,150	3,550
Due in 5-10 years	51	64
Due after 10 years	190,953	141,230
Total investment securities held-to-maturity	$ 193,554	$ 144,844