Loans Composition (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Total loans	[1]	$ 23,249,967	$ 22,940,429
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Total loans		7,579,384	7,398,044
Commercial real estate — owner occupied
Loans and Leases Receivable Disclosure [Line Items]
Total loans		942,811	920,443
Commercial and business lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans		8,522,194	8,318,487
Commercial real estate — investor
Loans and Leases Receivable Disclosure [Line Items]
Total loans		3,779,201	3,751,554
Real estate construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans		1,394,815	1,335,031
Commercial real estate lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans		5,174,016	5,086,585
Total commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans		13,696,210	13,405,072
Residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Total loans		8,277,479	8,277,712
Home equity
Loans and Leases Receivable Disclosure [Line Items]
Total loans		916,213	894,473
Other consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans		360,065	363,171
Total consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans		9,553,757	9,535,357
Bank Mutual
Loans and Leases Receivable Disclosure [Line Items]
Purchased credit-impaired loans		$ 2,000	$ 5,000

[1] (a) Includes $2 million and $5 million of purchased credit-impaired loans at June 30, 2019 and December 31, 2018, respectively.