Goodwill and Other Intangible Assets - Total Amortizing Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 1,070,027	$ 1,064,727
Accumulated Amortization	998,438	967,072
Net Carrying Amount	71,589	97,655
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	887,459	887,459
Accumulated Amortization	820,110	789,988
Net Carrying Amount	67,349	97,471
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	182,568	177,268
Accumulated Amortization	178,328	177,084
Net Carrying Amount	$ 4,240	$ 184