Balance Sheet Offsetting - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
US Treasury [Member]
Offsettting Assets and Liabilities [Line Items]
Fair value of treasury securities	$ 462.4	$ 461.3
Mortgage-Backed Securities [Member]
Offsettting Assets and Liabilities [Line Items]
Fair value of treasury securities	$ 457.5	$ 457.0

Fair value of securities pledged as collateral against derivative liabilities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226013-175313

Name:	us-gaap_DerivativeLiabilityFairValueOfCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant