Note Debt Securities Held-to-maturity (Amortized cost, gross unrealized gains and losses, approximate fair value, weighted average yield and contractual maturities of debt securities HTM) (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Held To Maturity Securities Member
Schedule of Held-to-maturity Securities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Held-to-maturity	$ 89.1	$ 92.8