Loans, Loan Composition (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 20,784,991	$ 20,054,716
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	6,399,693	6,489,014
Commercial real estate — owner occupied
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	802,209	897,724
Commercial and business lending
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,201,902	7,386,738
Commercial real estate — investor
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,315,254	3,574,732
Real estate construction
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,451,684	1,432,497
Commercial real estate lending
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,766,938	5,007,229
Total commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	11,968,840	12,393,967
Residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,546,534	6,332,327
Home equity
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	883,804	934,443
Other consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	385,813	393,979
Total consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 8,816,151	$ 7,660,749