Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Deferred compensation expense	$ 56,989	$ 51,169
Allowance for loan and lease losses and nonperforming assets	38,245	37,185
Investment securities	45,846	35,819
State income taxes	5,378	5,155
Total deferred income tax assets before valuation allowance	146,458	129,328
Valuation allowance	(1,901)	(1,834)
Total deferred income tax assets after valuation allowance	144,557	127,494
Liabilities:
Leases	(10,597)	(13,634)
Deferred income	(15,471)	(6,262)
Intangible assets	(788)	(969)
Other	(9,852)	(8,130)
Total deferred income tax liabilities	(36,708)	(28,995)
Net deferred income tax assets	$ 107,849	$ 98,499