Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 1,060	$ 1,349
Due after one through five years	8,850	5,545
Due after five through ten years	90,059	70,777
Due after ten years	1,281,775	1,097,451
Total debt securities with a single maturity date	1,381,744	1,175,122
Total investment securities	2,334,464	1,807,389
Fair Value
Due in one year or less	1,067	1,353
Due after one through five years	9,176	5,764
Due after five through ten years	95,333	75,223
Due after ten years	1,363,038	1,182,110
Total debt securities with a single maturity date	1,468,614	1,264,450
Total investment securities	2,426,900	1,919,119
Amortized Cost
Due in one year or less	8,405	9,712
Due after one through five years	23,079	22,241
Due after five through ten years	147,663	115,408
Due after ten years	746,456	505,153
Total debt securities with a single maturity date	925,603	652,514
Amortized Cost	1,721,659	1,227,668
Fair Value
Due in one year or less	8,461	9,755
Due after one through five years	24,351	23,190
Due after five through ten years	152,770	121,333
Due after ten years	769,606	533,079
Total debt securities with a single maturity date	955,188	687,357
Investment securities held to maturity	1,760,847	1,280,293
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	952,720	632,267
Fair Value
Without single maturity date	958,286	654,669
Total investment securities	958,286	654,669
Amortized Cost
Without single maturity date	796,056	575,154
Amortized Cost	796,056	575,154
Fair Value
Without single maturity date	805,659	592,936
Investment securities held to maturity	$ 805,659	$ 592,936