Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	37,852	22,136
After five years but within ten years	239,845	194,197
After ten years	2,172,568	1,693,150
Amortized Cost	2,450,265	1,909,483
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	39,985	22,207
After five years but within ten years	251,874	194,376
After ten years	2,237,198	1,728,331
Available-for-sale securities, fair value total	2,529,057	1,944,914
Held to Maturity, Amortized Cost
Within one year	1,144	2,649
After one year but within five years	972	4,239
After five years but within ten years	35,967	35,288
After ten years	73,664	91,425
Amortized Cost	111,747	133,601
Held to Maturity, Fair Value
Within one year	1,154	2,653
After one year but within five years	990	4,270
After five years but within ten years	37,317	35,967
After ten years	76,960	93,735
Held-to-maturity securities, fair value total	$ 116,421	$ 136,625