Share-Based Compensation - Compensation Expense Included in Non-Interest Expense and Related Income Tax Benefits (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 322	$ 343	$ 952	$ 1,131
Income tax benefit	23	46	70	161
Restricted Stock And Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	4,843	4,046	14,192	10,368
Income tax benefit	1,017	1,416	2,980	3,629
Phantom Stock and Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 2,375	$ 2,747	$ 7,388	$ 8,689