Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2017 USD ($) quarter shares	Sep. 30, 2016 USD ($) shares	Dec. 31, 2016 USD ($)	Oct. 27, 2016 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount					$ 100,000
Stock Repurchase Program, Period in Force		2 years
Treasury stock, shares, acquired | shares		1,135,435	0
Treasury Stock, Value, Acquired, Cost Method		$ 100,042	$ 0
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%
Corporation's aggregate subordinated notes	136,170	$ 136,170		$ 136,127
Maximum dividends available without prior regulatory approval	480,200	$ 480,200
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
Floating Rate Subordinated Notes [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Corporation's aggregate subordinated notes	$ 100,000	$ 100,000
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