Business Combination - Additional Information (Details) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Mar. 22, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Total Assets	$ 171,079		$ 141,590	$ 171,079	$ 141,590		$ 146,069
Full-service Banking Centers	1,143			1,143
Total noninterest expense	$ 1,159		972	$ 3,499	2,984
Class B Preferred stock, Series A
Business Acquisition [Line Items]
Preferred stock, issued | shares	200,000			200,000
Preferred stock, liquidation preference | $ / shares	$ 1,000			$ 1,000
MB Financial, Inc.
Business Acquisition [Line Items]
Total consideration paid		$ 3,608
Total Assets							$ 20,000
Full-service Banking Centers							86
Fair value of noncontrolling interest in acquiree						$ 197
Goodwill						1,815
Tax deductible goodwill						15
Total noninterest expense	$ 28		$ 1	$ 213	$ 3
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
Cash consideration transferred per share | $ / shares						$ 5.54
Share consideration transferred | shares						1.45
Total value per share | $ / shares						$ 42.49
Closing price of common stock | $ / shares						$ 25.48
Concractual balances on PCI	872			872
Fair value for PCI	$ 637			$ 637
Locations							91