Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill	$ 898,434,000	$ 898,434,000		$ 808,322,000	$ 898,434,000	$ 808,322,000		$ 808,322,000	$ 808,726,000
Goodwill acquired during the year					92,844,000	0
Purchase accounting adjustments					(2,732,000)	(404,000)
Impairment losses	0	0	$ 0
Net intangible assets	90,012,000	94,550,000		87,560,000	90,012,000	87,560,000
Additions due to acquisitions	0	19,267,000		$ 0	19,267,000	0
Remainder of fiscal year	17,100,000				17,100,000
2021	15,900,000				15,900,000
2022	14,000,000.0				14,000,000.0
2023	12,300,000				12,300,000
2024	11,100,000				$ 11,100,000
Min
Goodwill [Line Items]
Weighted average useful life					6 years
Max
Goodwill [Line Items]
Weighted average useful life					11 years
Core Deposits
Goodwill [Line Items]
Net intangible assets	86,900,000				$ 86,900,000			$ 83,300,000
Customer Relationships
Goodwill [Line Items]
Net intangible assets	3,100,000				$ 3,100,000
Opus Bank
Goodwill [Line Items]
Goodwill							$ 90,112,000
Purchase accounting adjustments	$ (2,700,000)
Opus Bank | Core Deposits
Goodwill [Line Items]
Additions due to acquisitions		16,100,000
Opus Bank | Customer Relationships
Goodwill [Line Items]
Additions due to acquisitions		$ 3,200,000
Grandpoint Capital, Inc.
Goodwill [Line Items]
Purchase accounting adjustments						$ (404,000)