Business Combination (Business Combination - Additional Information) (Detail) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 22, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Business Acquisition
Total Assets	$ 167,853	$ 167,853	$ 141,383		$ 146,069
Full-service Banking Centers	1,207	1,207
Total noninterest expense		$ 1,097	$ 1,010
MB Financial, Inc.
Business Acquisition
Total consideration paid		3,608
Total Assets					$ 20,000
Full-service Banking Centers					86
Fair value of noncontrolling interest in acquiree				$ 197
Goodwill				1,843
Tax deductible goodwill				15
Total noninterest expense		$ 76
Contractual balances on the purchased non credit impaired loans and leases				12,900
Fair value of the purchased non credit impaired loans and leases				$ 12,700
Acquisition date through period end, net income	$ 13
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
MB Financial, Inc. | Net Interest Income
Business Acquisition
Acquisition date through period end, revenue	16
MB Financial, Inc. | Noninterest Income
Business Acquisition
Acquisition date through period end, revenue	$ 12