Carrying Amounts and Market Values of Securities and Related Repurchase Liabilities and Weighted Average Interest Rates of Repurchase Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Securities Market Value	$ 1,234,482
Repurchase Liabilities	$ 1,249,370	$ 1,400,000
Weighted Average Interest Rate	1.10%
On Demand [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Market Value	$ 1,004
Repurchase Liabilities	$ 1,000
Weighted Average Interest Rate	2.49%
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Market Value	$ 1,233,478
Repurchase Liabilities	$ 1,248,370
Weighted Average Interest Rate	1.10%