Loans - Composition (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total loans	$ 22,810,491	$ 20,784,991
Purchased credit-impaired loans	233,078	247,575
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	6,756,983	6,399,693
Purchased credit-impaired loans	128,841	142,244
Commercial real estate — owner occupied
Loans and Leases Receivable Disclosure [Line Items]
Total loans	900,913	802,209
Purchased credit-impaired loans	22,489	26,234
Commercial and business lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans	7,657,896	7,201,902
Purchased credit-impaired loans	151,330	168,478
Commercial real estate — investor
Loans and Leases Receivable Disclosure [Line Items]
Total loans	4,077,671	3,315,254
Purchased credit-impaired loans	20,387	19,118
Real estate construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans	1,579,778	1,451,684
Purchased credit-impaired loans	458	467
Commercial real estate lending
Loans and Leases Receivable Disclosure [Line Items]
Total loans	5,657,449	4,766,938
Total commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	13,315,345	11,968,840
Residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Total loans	8,197,223	7,546,534
Purchased credit-impaired loans	48,863	47,486
Home equity
Loans and Leases Receivable Disclosure [Line Items]
Total loans	923,470	883,804
Purchased credit-impaired loans	10,974	10,891
Other consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	374,453	385,813
Purchased credit-impaired loans	1,066	1,135
Total consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	9,495,146	$ 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	$ 15,000