Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 24, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) quarter shares	Sep. 30, 2017 USD ($) shares	Dec. 31, 2017 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares		0	1,134,966	601	1,135,435
Treasury Stock, Value, Acquired, Cost Method			$ 100,000	$ 70	$ 100,042
Preferred stock value issued include for regulatory tier one capital calculations		$ 144,500		$ 144,500		$ 144,500
Preferred stock dividend rate percentage				5.375%		5.375%
Qualified subordinated debt		98,669		$ 98,669		$ 98,552
Maximum dividends available without prior regulatory approval		569,700		$ 569,700
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter				20
Trust Preferred Securities Tier One Excluded Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities				$ 133,000		133,000
Stock Repurchase 2017 Plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 150,000
Stock Repurchase Program, Period in Force	2 years
Stock Repurchase 2017 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares				0	0
Stock Repurchase 2016 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares			1,134,966
Treasury Stock, Value, Acquired, Cost Method			$ 100,000
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Qualified subordinated debt		$ 100,000		$ 100,000		$ 100,000