Loans - Risk Category of Loans (Details) - Non-PCI Loans - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 13,873,233	$ 11,920,880
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	3,243,622	2,549,065
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	9,607,279	8,561,851
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,022,332	809,964
Pass
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	13,598,670	11,631,645
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	3,059,312	2,375,689
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	9,518,179	8,447,865
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,021,179	808,091
Special Mention
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	82,434	110,440
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	52,555	62,071
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	29,334	48,009
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	545	360
Substandard
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	142,586	164,045
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	82,212	96,555
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	59,766	65,977
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	608	1,513
Doubtful
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	49,543	14,750
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	49,543	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