Long-Term Debt - Summary of Long-term Debt (Parenthetical) (Detail)	Dec. 31, 2017
Medium-term Notes [Member]
Debt Instrument [Line Items]
Weighted-average interest rate	2.51%
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Weighted-average interest rate	1.83%
Bank Notes [Member]
Debt Instrument [Line Items]
Weighted-average interest rate	1.86%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration