Investment Securities - Narrative (Details)	3 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) investmentSecurity	Dec. 31, 2019 USD ($) investmentSecurity	Dec. 31, 2018 USD ($)	Jan. 01, 2020 USD ($)
Investment Holdings [Line Items]
Accumulated other comprehensive income (loss)		$ 84,400,000	$ 30,100,000
Accumulated other comprehensive income (loss), net of tax		60,292,000	21,523,000
Available-for-sale securities acquired		2,715,499,000	889,516,000	$ 462,534,000
Available-for-sale securities, provision for credit losses		$ 0
OTTI on investment securities			$ 0	0
Available-for-sale and held-to-maturity securities in nonaccrual status | investmentSecurity		0
Available-for-sale and held-to-maturity securities purchased with deterioration in credit quality | investmentSecurity		0	0
Available-for-sale and held-to-maturity collateral dependant | investmentSecurity			0
Gross gains		$ 15,700,000	$ 10,300,000	1,600,000
Gross losses		1,800,000	1,800,000	208,000
Proceeds from sale of securities available-for-sale		766,500,000	551,800,000	407,000,000.0
Investment securities pledged		147,300,000	125,700,000
FHLB stock		17,300,000
FRB stock		74,400,000
Other stock		25,400,000
FHLB repurchases		17,300,000	$ 18,300,000	$ 24,900,000
Impairment loss on investments in FHLB, FRB and other stock		0
Allowance for credit losses on debt securities, available-for-sale		0			$ 0
Allowance for credit losses on debt securities, held-to-maturity		$ 0			$ 0
Opus
Investment Holdings [Line Items]
Available-for-sale securities acquired	$ 829,900,000
Allowance for credit losses of available-for-sale securities classified as purchase credit deteriorated	0
Opus | Financial Asset Acquired with Credit Deterioration
Investment Holdings [Line Items]
Available-for-sale securities acquired	$ 0