Junior Subordinated Debentures (Narrative) (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) trust Rate	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Subordinated Borrowing [Line Items]
Percentage ownership interest in subsidiary trusts	100.00%
Number of unconsolidated subsidiary trusts | trust	11
Common securities, approximate percentage of junior subordinated debentures	3.00%
Trust preferred securities, approximate percentage of junior subordinated debentures	97.00%
Junior subordinated debentures	$ 253,566	$ 253,566	$ 253,566
Junior Subordinated Debt
Subordinated Borrowing [Line Items]
Junior subordinated debentures	$ 253,566
Debt, weighted average interest rate (as a percent) | Rate	4.55%
Period of deferred payment, not to exceed	60 months
Tier two risk based capital	$ 245,500