Acquisitions and Dispositions - Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Nov. 01, 2019	Apr. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Goodwill	$ 3,065.5			$ 2,685.7	$ 2,411.4
United Financial [Member]
Assets:
Cash and cash equivalents		$ 594.9
Securities		381.3
Loans		5,535.5
Goodwill		204.3
Core deposit intangible		41.5
Premises and equipment		61.8
BOLI		196.5
Lease ROU assets		56.3
Mortgage servicing rights		10.6
REO		1.2
Other assets		136.2
Total assets		7,220.1
Liabilities:
Deposits		5,687.2
Borrowings		622.4
Lease liabilities		61.4
Other liabilities		128.5
Total liabilities		6,499.5
Total purchase price		$ 720.6
BSB Bancorp [Member]
Assets:
Cash and cash equivalents			$ 108.7
Securities			175.8
Loans			2,642.9
Goodwill			144.9
Core deposit intangible			39.5
Premises and equipment			8.3
BOLI			36.8
Other assets			29.8
Total assets			3,186.7
Liabilities:
Deposits			2,118.7
Borrowings			696.6
Other liabilities			46.9
Total liabilities			2,862.2
Total purchase price			$ 324.5