Goodwill and Core Deposit Intangibles (Tables)

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Change in Carrying Amount of Goodwill

	Year Ended December 31,
	2020		2019
Net carrying value at beginning of period	$	621.6	$	546.7
Acquisitions and measurement period adjustments	—		74.9
Net carrying value at end of period	$	621.6	$	621.6

Schedule of Activity of Identifiable Core Deposit Intangibles

	Year Ended December 31,
	2020		2019
Gross CDI, beginning of period	$	106.0	$	89.7
Established through acquisitions	—		16.6
Reductions due to sale of accounts	—		(0.3)
Accumulated amortization	(54.8)		(43.9)
Net CDI, end of period	$	51.2	$	62.1

Schedule of Finite-Lived Intangible Assets, Future Amortization Expense

The following table provides estimated future CDI amortization expense:

Years ending December 31,
2021	$	9.9
2022	9.0
2023	8.2
2024	7.3
2025	6.5
Thereafter	10.3
Total	$	51.2