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, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 66,124	$ 0
Due in 1-5 years	191,364	24,464
Due in 5-10 years	547,813	509,832
Due after 10 years	5,833,976	1,962,708
Amortized Cost	6,639,277	2,497,004
Fair value
Due in one year or less	66,112	0
Due in 1-5 years	192,065	24,375
Due in 5-10 years	555,523	503,768
Due after 10 years	5,906,301	1,941,922
Total investment securities available-for-sale	6,720,001	2,470,065
Amortized cost
Due in 1-5 years	3,550	2,400
Due in 5-10 years	58	430
Due after 10 years	135,837	146,022
Amortized Cost	139,445	148,852
Fair value
Due in 1-5 years	3,550	2,400
Due in 5-10 years	64	432
Due after 10 years	141,230	146,435
Fair Value	$ 144,844	$ 149,267