Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Allowance for loan losses	$ 34,885	$ 47,485
Depreciation	8,336	12,432
Employee benefits	10,596	16,121
Investment securities, including other-than-temporary impairment losses	5,021	17,272
Net operating loss carryforwards	30,658	46,667
Purchase accounting	18,819	33,172
Other	21,930	22,183
Total deferred tax assets	130,245	195,332
Deferred tax liabilities:
Pension plans	18,912	24,575
Other investments	13,234	20,831
Deferred income	37,952	0
Other	12,651	20,418
Total deferred tax liabilities	82,749	65,824
Net deferred tax asset (included in other assets)	$ 47,496	$ 129,508