Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized cost within one year	$ 28,888	$ 44,206
Amortized cost one to five years	133,651	61,594
Amortized cost for five to ten years	320,561	136,857
Amortized cost for after ten years	828,788	589,200
Available for sale	1,311,888	831,857
Amortized Cost	1,655,190	1,090,363
Within one year, amortized cost	4,612	7,302
One to five years, amortized cost	45,148	42,742
Five to ten years, amortized cost	185,847	82,087
After ten years, amortized cost	181,925	25,290
Total amortized cost	417,532	157,421
Investment securities, held to maturity	769,240	168,676
Fair value within one year	28,885	44,192
Fair value for one to five years	134,899	63,006
Fair value for five to ten years	322,940	145,102
Fair value for after ten years	836,743	615,473
Total fair value	1,323,467	867,773
Investment securities, available for sale	1,669,634	1,134,025
Within one year, fair value	4,666	7,327
One to five years, fair value	46,573	44,358
Five to ten years, fair value	188,962	88,300
After ten years, fair value	181,094	28,471
Total fair value	421,295	168,456
Fair Value	768,099	179,990
Federal agency collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Available for sale	73,922	144,022
Amortized Cost	73,922	144,022
Investment securities, held to maturity	69,537	2,661
Investment securities, available for sale	75,525	147,453
Fair Value	68,754	2,697
Federal agency mortgage-backed pools
Schedule of Available-for-sale Securities [Line Items]
Available for sale	236,432	114,484
Amortized Cost	236,432	114,484
Investment securities, held to maturity	215,873	8,594
Investment securities, available for sale	237,705	118,799
Fair Value	212,498	8,837
Private labeled mortgage-backed pools
Schedule of Available-for-sale Securities [Line Items]
Available for sale	32,948	0
Amortized Cost	32,948
Investment securities, held to maturity	66,298	0
Investment securities, available for sale	32,937	0
Fair Value	$ 65,552	$ 0

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