Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 2,421	$ 1,349
Due after one through five years	13,087	5,545
Due after five through ten years	100,122	70,777
Due after ten years	1,367,250	1,097,451
Total debt securities with a single maturity date	1,482,880	1,175,122
Total investment securities	2,307,977	1,807,389
Fair Value
Due in one year or less	2,428	1,353
Due after one through five years	14,068	5,764
Due after five through ten years	105,457	75,223
Due after ten years	1,429,488	1,182,110
Total debt securities with a single maturity date	1,551,441	1,264,450
Total investment securities	2,374,578	1,919,119
Amortized Cost
Due in one year or less	8,718	9,712
Due after one through five years	28,789	22,241
Due after five through ten years	176,603	115,408
Due after ten years	874,315	505,153
Total debt securities with a single maturity date	1,088,425	652,514
Amortized Cost	2,071,183	1,227,668
Fair Value
Due in one year or less	8,766	9,755
Due after one through five years	30,589	23,190
Due after five through ten years	180,790	121,333
Due after ten years	882,238	533,079
Total debt securities with a single maturity date	1,102,383	687,357
Investment securities held to maturity	2,086,555	1,280,293
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	825,097	632,267
Fair Value
Without single maturity date	823,137	654,669
Total investment securities	823,137	654,669
Amortized Cost
Without single maturity date	982,758	575,154
Amortized Cost	982,758	575,154
Fair Value
Without single maturity date	984,172	592,936
Investment securities held to maturity	$ 984,172	$ 592,936