Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 63,391	$ 66,124
Due in 1-5 years	182,753	191,364
Due in 5-10 years	558,013	547,813
Due after 10 years	6,147,126	5,833,976
Amortized Cost	6,951,283	6,639,277
Fair value
Due in one year or less	63,440	66,112
Due in 1-5 years	184,351	192,065
Due in 5-10 years	588,103	555,523
Due after 10 years	6,383,479	5,906,301
Total investment securities available-for-sale	7,219,373	6,720,001
Amortized cost
Due in 1-5 years	3,550	3,550
Due in 5-10 years	52	58
Due after 10 years	126,499	135,837
Amortized Cost	130,101	139,445
Fair value
Due in 1-5 years	3,550	3,550
Due in 5-10 years	58	64
Due after 10 years	137,499	141,230
Fair Value	$ 141,107	$ 144,844