Business Combination - Additional Information (Details) $ / shares in Units, $ in Millions				12 Months Ended
		Jan. 01, 2020 USD ($)	Mar. 22, 2019 USD ($) $ / shares shares	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) branch location	Aug. 26, 2019 $ / shares shares
Business Acquisition [Line Items]
Total assets				$ 204,680	$ 169,369
Goodwill				4,258	4,252	$ 2,478
Total noninterest expense	[1]			4,718	4,660	$ 3,958
Increase in purchased financial assets with credit deterioration		$ 33
Unpaid principal balance as noncredit discount		$ 87
Class C Preferred stock
Business Acquisition [Line Items]
Fixed dividend rate (as a percent)			6.00%
Class B Preferred stock, Series A
Business Acquisition [Line Items]
Fixed dividend rate (as a percent)							6.00%
Preferred stock, issued (in shares) | shares							200,000
Preferred stock, liquidation preference per share (in dollars per share) | $ / shares							$ 1,000
MB Financial, Inc.
Business Acquisition [Line Items]
Total consideration paid			$ 3,608
Total assets			$ 20,000
Full-service banking centers | branch						86
Number of locations | location						91
Percentage of common stock acquired			100.00%
Share consideration transferred | shares			1.45
Cash consideration transferred per share (in dollars per share) | $ / shares			$ 5.54
Total value per share (in dollars per share) | $ / shares			42.49
Closing price of common stock (in dollars per share) | $ / shares			$ 25.48
Fair value of noncontrolling interest in acquiree			$ 197
Percentage of voting equity interest			95.00%
Goodwill			$ 1,786
Tax deductible goodwill			$ 15
Tax deductible goodwill, period			15 years
Total noninterest expense				$ 16	$ 222
Contractual balances on the purchased non credit impaired loans and leases			$ 12,700
Fair value of the purchased non credit impaired loans and leases			$ 12,500

[1] During the first quarter of 2020, certain noninterest income and noninterest expense line items were reclassified to better align disclosures to business activities. These reclassifications were retrospectively applied to all prior periods presented. Total noninterest income and noninterest expense did not change as a result of these reclassifications.