Investment Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Security	Dec. 31, 2013 Security	Dec. 31, 2012	Sep. 30, 2012
Investment Securities [Line Items]
Held to maturity, fair value	$ 1,778,316,000	$ 1,731,737,000
Available for sale	886,970,000	829,692,000
Fair value of trading securities	14,233,000	14,264,000
Number of security positions in the securities held to maturity portfolio in an unrealized loss position	57	133
Fair value of investments held to maturity pledged as collateral	854,900,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years	7 years 3 months 18 days
Number of security positions in the securities available for sale portfolio in an unrealized loss position	96	99
Gross unrealized losses	12,331,000	46,262,000
Amortized cost	889,958,000	866,904,000
Gross unrealized losses	12,331,000	46,262,000
Fair value of securities available for sale pledged as collateral	414,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years	5 years 5 months
Investments held	0
Credit factors risk percentage	25.00%
Recognized impairment charges		722,000
Gross gains on sales transactions	746,000	14,098,000	6,621,000
Gross gains	744,000	14,054,000	5,571,000
Gross losses on the early redemption			5,300,000
Interest income on trading securities	1,200,000	1,400,000	1,800,000
Single Issuer Trust Preferred Security [Member] | Non-Rated [Member]
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position	4
Single-Issuer Bank Trust Preferred Securities [Member]
Investment Securities [Line Items]
Fair value of trading securities	14,200,000	14,300,000
Trust preferred securities [Member]
Investment Securities [Line Items]
Impairment charge part of combined amortized cost of the securities	0	0		4,500,000
Three Private Label Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Other-than-temporarily impaired mortgage-backed securities, amortized cost	13,700,000
Other-than-temporarily impaired mortgage-backed securities, fair value	14,600,000
Pooled Trust Preferred Securities [Member]
Investment Securities [Line Items]
Available for sale	8,700,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position	2	3
Amortized cost	10,800,000
Other-than-temporarily impaired trust preferred securities, amortized cost	5,400,000
Other-than-temporarily impaired trust preferred securities, fair value	4,700,000
Gross gains on sales transactions		10,700,000
Pooled Trust Preferred Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position	1
Gross unrealized losses	1,400,000
Second Pooled Trust Preferred Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Gross unrealized losses	698,000
Obligations of states and political subdivisions [Member]
Investment Securities [Line Items]
Held to maturity, fair value	500,018,000	545,886,000
Available for sale	44,051,000	37,700,000
Gross unrealized losses	576,000	2,163,000
Amortized cost	44,500,000	39,112,000
Gross unrealized losses	576,000	2,163,000
Obligations of states and political subdivisions [Member] | Other Than Temporarily Impaired Securities [Member]
Investment Securities [Line Items]
Available for sale	19,200,000
Amortized cost	18,400,000
Gross unrealized losses	1,300,000
Investment carrying value	$ 544,100,000
Investment carrying value percentage	55.00%