Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Aug. 01, 2019
Business Acquisition [Line Items]
Goodwill		$ 1,313,046,000	$ 1,299,878,000	$ 1,158,289,000
Goodwill impairment charge		0	0
Impairment of other intangible assets		0	0
Amortization expense for intangible assets	$ 3,400,000	22,000,000.0	11,700,000
Other intangible assets, net		$ 146,377,000	$ 168,368,000
Core Deposits and Customer Relationships
Business Acquisition [Line Items]
Other intangible assets, net				$ 159,500,000