Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 45,229	$ 66,124
Due in 1-5 years	171,134	191,364
Due in 5-10 years	664,743	547,813
Due after 10 years	6,306,586	5,833,976
Amortized Cost	7,187,692	6,639,277
Fair value
Due in one year or less	45,518	66,112
Due in 1-5 years	176,145	192,065
Due in 5-10 years	703,227	555,523
Due after 10 years	6,521,273	5,906,301
Total investment securities available-for-sale	7,446,163	6,720,001
Amortized cost
Due in one year or less	400	0
Due in 1-5 years	3,150	3,550
Due in 5-10 years	49	58
Due after 10 years	166,710	135,837
Amortized Cost	170,309	139,445
Fair value
Due in one year or less	400	0
Due in 1-5 years	3,150	3,550
Due in 5-10 years	55	64
Due after 10 years	176,759	141,230
Fair Value	$ 180,364	$ 144,844