Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands		3 Months Ended			12 Months Ended
	Oct. 24, 2017 USD ($)	Mar. 31, 2018 USD ($) quarter shares	Sep. 30, 2017 USD ($) shares	Mar. 31, 2017 USD ($) shares	Dec. 31, 2017 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares		0		469
Treasury Stock, Value, Acquired, Cost Method		$ 0		$ 42
Preferred stock value issued include for regulatory tier one capital calculations		$ 144,500			$ 144,500
Preferred stock dividend rate percentage		5.375%			5.375%
Qualified subordinated debt		$ 98,591			$ 98,552
Maximum dividends available without prior regulatory approval		$ 446,700
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