Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	100.00%	100.00%
Percentage of adjusted specific issuer evaluation recoveries	100.00%	100.00%
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	0.00%	0.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	50.00%	50.00%
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 recoveries	0.00%	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 recoveries	25.00%	25.00%