Financial Instruments with Off-Balance Sheet Risk - Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	$ 2,929,329	$ 2,657,279
Standby letters of credit and guarantees on industrial revenue bonds
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	20,662	12,992
Commitments to extend credit
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	2,908,667	2,644,287
Commitments to extend credit | Consumer real estate and other
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	1,627,960	1,484,065
Commitments to extend credit | Commercial
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	1,127,368	1,033,973
Commitments to extend credit | Leasing and equipment finance
Financial instruments with off-balance sheet risk
Financial instruments with off-balance sheet risk	$ 153,339	$ 126,249