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, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 66,096	$ 66,124
Due in 1-5 years	188,565	191,364
Due in 5-10 years	551,643	547,813
Due after 10 years	5,986,931	5,833,976
Amortized Cost	6,793,235	6,639,277
Fair value
Due in one year or less	66,021	66,112
Due in 1-5 years	189,294	192,065
Due in 5-10 years	569,940	555,523
Due after 10 years	6,199,969	5,906,301
Total investment securities available-for-sale	7,025,224	6,720,001
Amortized cost
Due in 1-5 years	3,550	3,550
Due in 5-10 years	55	58
Due after 10 years	132,014	135,837
Amortized Cost	135,619	139,445
Fair value
Due in 1-5 years	3,550	3,550
Due in 5-10 years	61	64
Due after 10 years	142,363	141,230
Fair Value	$ 145,974	$ 144,844