Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill	$ 901,166,000	$ 901,166,000	$ 808,322,000	$ 901,166,000	$ 808,322,000		$ 808,322,000	$ 808,726,000
Goodwill acquired during the year				92,844,000	0
Purchase accounting adjustments				0	(404,000)
Impairment losses				0	0
Net intangible assets	94,550,000	79,349,000	91,840,000	94,550,000	91,840,000
Additions due to acquisitions	19,267,000	$ 0	$ 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	91,400,000			$ 91,400,000			$ 83,300,000
Customer Relationships
Goodwill [Line Items]
Net intangible assets	3,200,000			$ 3,200,000
Opus Bank
Goodwill [Line Items]
Goodwill						$ 92,800,000
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)