Goodwill and Other Intangible Assets - Carrying amount (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Core deposit intangible	$ 36,162		$ 32,802
Less: accumulated amortization	(21,106)		(17,980)
Net core deposit intangible	15,056		14,822
Goodwill	369,018	$ 327,425	327,425	$ 327,425	$ 327,425	$ 307,112
Total goodwill and other intangible assets, net	$ 384,074		$ 342,247