Summary of Significant Accounting Policies - CECL Adjustments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ (52,162)	$ (168,098)	$ (145,387)		$ (50,814)	$ (50,059)	$ (49,026)
Deferred tax assets, net	27,282	42,047
Remaining purchase discount on loans	(50,958)
Reserve for unfunded commitments	946	20,035	$ 17,335
Retained earnings	617,355	$ 596,779
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	(42,484)			$ (42,484)
Deferred tax assets, net	12,305
Remaining purchase discount on loans	5,469
Reserve for unfunded commitments	10,389
Retained earnings	(35,099)
Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	(94,646)
Deferred tax assets, net	39,587
Remaining purchase discount on loans	(45,489)
Reserve for unfunded commitments	11,335
Retained earnings	$ 582,256