Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended		15 Months Ended
	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020	Jun. 01, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 898,434		$ 901,312	$ 898,434	$ 901,312	$ 898,569		$ 808,322
Purchase accounting adjustments to goodwill			2,743	(2,732)
Goodwill acquired during the year			0	92,844
Net intangible assets	90,012	$ 77,363	$ 73,451	90,012	73,451	85,500
Remainder of fiscal year						15,900
2022						14,000
2023						12,300
2024						11,100
2025						10,000
Minimum
Goodwill [Line Items]
Weighted average useful life			6 years
Maximum
Goodwill [Line Items]
Weighted average useful life			11 years
Core Deposits
Goodwill [Line Items]
Net intangible assets			$ 70,700		70,700	82,500
Customer Relationships
Goodwill [Line Items]
Net intangible assets			$ 2,800		2,800	$ 3,000
Opus Bank
Goodwill [Line Items]
Goodwill							$ 92,990
Purchase accounting adjustments to goodwill	$ (2,700)	$ 2,700			$ 146
Goodwill acquired during the year				$ 92,800