Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 45,902	$ 45,912
Due in 1-5 years	160,915	163,346
Due in 5-10 years	770,039	681,135
Due after 10 years	7,420,117	7,150,780
Amortized Cost	8,396,973	8,041,173
Fair value
Due in one year or less	46,020	46,163
Due in 1-5 years	163,885	168,125
Due in 5-10 years	788,800	720,239
Due after 10 years	7,405,083	7,350,196
Total investment securities available-for-sale	8,403,788	8,284,723
Amortized cost
Due in one year or less	400	400
Due in 1-5 years	2,150	2,150
Due in 5-10 years	43	46
Due after 10 years	207,185	181,763
Amortized Cost	209,778	184,359
Fair value
Due in one year or less	400	400
Due in 1-5 years	2,150	2,150
Due in 5-10 years	48	51
Due after 10 years	209,813	190,953
Fair Value	$ 212,411	$ 193,554