Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	100.00%
Percentage of adjusted specific issuer evaluation recoveries	100.00%
Pooled Trust Preferred Securities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	0.00%
Percentage of adjusted specific issuer evaluation recoveries	0.00%
Pooled Trust Preferred Securities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	50.00%
Percentage of adjusted specific issuer evaluation recoveries	25.00%