Investment Securities - Summary of Transfers between Level 1 and Level 2 (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers from Level 2 to Level 1	$ 2,416	$ 11,032
Obligations of U.S. Government Sponsored Enterprises [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers from Level 2 to Level 1	995
Corporate Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers from Level 2 to Level 1	$ 1,421	5,737
Foreign Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers from Level 2 to Level 1		$ 5,295