Loans - Risk Category of Loans (Details) - Non-PCI Loans - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 15,677,438	$ 14,625,032
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	3,918,894	3,590,375
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	10,470,127	9,912,309
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,288,417	1,122,348
Pass
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	15,382,068	14,349,491
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	3,733,252	3,399,426
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	10,362,538	9,828,744
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,286,278	1,121,321
Special Mention
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	105,313	63,103
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	40,412	31,996
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	63,116	30,892
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,785	215
Substandard
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	119,771	144,842
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	75,853	92,320
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	43,564	51,710
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	354	812
Doubtful
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	70,286	67,596
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	69,377	66,633
Doubtful | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	909	963
Doubtful | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 0	$ 0