Loans Composition (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total loans	$ 22,976,786	$ 20,784,991
Purchased credit-impaired loans	211,687	247,575
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	7,109,796	6,399,693
Purchased credit-impaired loans	110,993	142,244
Commercial real estate — owner occupied
Loans and Leases Receivable Disclosure [Line Items]
Total loans	888,330	802,209
Purchased credit-impaired loans	20,255	26,234
Commercial and business lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans	7,998,126	7,201,902
Purchased credit-impaired loans	131,248	168,478
Commercial real estate — investor
Loans and Leases Receivable Disclosure [Line Items]
Total loans	3,996,415	3,315,254
Purchased credit-impaired loans	16,381	19,118
Real estate construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans	1,487,159	1,451,684
Purchased credit-impaired loans	448	467
Commercial real estate lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans	5,483,574	4,766,938
Total commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	13,481,700	11,968,840
Residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Total loans	8,207,253	7,546,534
Purchased credit-impaired loans	51,469	47,486
Home equity
Loans and Leases Receivable Disclosure [Line Items]
Total loans	911,363	883,804
Purchased credit-impaired loans	11,092	10,891
Other consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	376,470	385,813
Purchased credit-impaired loans	1,049	1,135
Total consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	9,495,086	$ 8,816,151
Bank Mutual [Member]
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income, Purchased Credit Impaired Loans And Covered Loans	$ 16,000