Maturities of Collateral Underlying Reverse Repurchase Agreements and Securities Lending Transactions (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	$ 10,139	$ 36,337
Securities lending transactions	188,215	203,909
Total Borrowings	198,354	240,246
Gross amount of recognized liabilities for reverse repurchase agreements and securities lending included on the Balance Sheets	203,777	246,417
Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	10,139	36,337
Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	22,360	34,493
Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	99	405
Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	188,215	203,909
Total Borrowings	188,215	203,909
Maturity Overnight and Continuous | Corporate securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	22,360	34,493
Maturity Overnight and Continuous | Equity AFS securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	99	405
Maturity up to 30 Days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	10,139	36,337
Total Borrowings	10,139	36,337
Maturity up to 30 Days | Residential mortgage-backed securities
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	10,139	36,337
US Government Corporations and Agencies Securities
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	165,756	169,011
US Government Corporations and Agencies Securities | Maturity Overnight and Continuous
Assets Sold Under Agreements To Repurchase [Line Items]
Securities lending transactions	$ 165,756	$ 169,011