Loans - Risk Category of Loans (Details) - Non-PCI Loans - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 13,127,871	$ 11,920,880
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	3,021,524	2,549,065
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	9,109,645	8,561,851
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	996,702	809,964
Pass
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	12,867,408	11,631,645
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,853,692	2,375,689
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	9,019,309	8,447,865
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	994,407	808,091
Special Mention
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	98,092	110,440
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	62,369	62,071
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	35,200	48,009
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	523	360
Substandard
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	113,003	164,045
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	56,095	96,555
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	55,136	65,977
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,772	1,513
Doubtful
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	49,368	14,750
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	49,368	14,750
Doubtful | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	0	0
Doubtful | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 0	$ 0