Maturities of Collateral Underlying Reverse Repurchase Agreements and Securities Lending Transactions (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	$ 36,337	$ 46,401
Securities lending transactions	203,909	194,996
Total Borrowings	240,246	241,397
Gross amount of recognized liabilities for reverse repurchase agreements and securities lending included on the Balance Sheets	246,417	246,049
Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	36,337	46,401
Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	34,493	40,774
Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	405	9,517
Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	203,909	194,996
Total Borrowings	203,909	194,996
Maturity Overnight and Continuous | Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	34,493	40,774
Maturity Overnight and Continuous | Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	405	9,517
Maturity up to 30 Days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	36,337	46,401
Total Borrowings	36,337	46,401
Maturity up to 30 Days | Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	36,337	46,401
US Government Corporations and Agencies Securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	169,011	144,705
US Government Corporations and Agencies Securities | Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	$ 169,011	$ 144,705