Investment Securities - Additional Information (Detail)		3 Months Ended	12 Months Ended
		Mar. 31, 2015 USD ($) Security	Dec. 31, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security	Dec. 31, 2013 USD ($)
Investment Securities [Line Items]
Held to maturity, fair value			$ 1,596,385,000	$ 1,778,316,000
Available for sale			1,506,861,000	886,970,000
Loss on Sale of Investments			947,000	2,000	$ 44,000
Fair value of trading securities			$ 0	$ 14,233,000
Number of security positions in the securities held to maturity portfolio in an unrealized loss position | Security			74	57
Fair value of investments held to maturity pledged as collateral			$ 943,500,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			6 years 7 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position | Security			291	96
Gross unrealized losses			$ 16,312,000	$ 12,331,000
Amortized cost			1,516,937,000	889,958,000
Gross unrealized losses			16,312,000	12,331,000
Fair value of securities available for sale pledged as collateral			$ 489,800,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			8 years 6 months
Gross gains on sales transactions			$ 3,274,000	746,000	14,098,000
Gross gains			2,327,000	744,000	14,054,000
Gross losses on the early redemption					3,300,000
Amortized Cost			25,900,000
Interest income on trading securities			$ 213,000	1,200,000	1,800,000
Corporate and other debt securities [Member]
Investment Securities [Line Items]
Held to maturity, fair value		$ 9,800,000
Number of held-to-maturity securities sold (in security) | Security		1
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 | Security			4
Single-Issuer Bank Trust Preferred Securities [Member]
Investment Securities [Line Items]
Fair value of trading securities				14,200,000
Trust preferred securities [Member]
Investment Securities [Line Items]
Impairment charge part of combined amortized cost of the securities			$ 0	0	0
Three Private Label Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Other-than-temporarily impaired mortgage-backed securities, amortized cost			11,900,000
Other-than-temporarily impaired mortgage-backed securities, fair value			11,700,000
Pooled Trust Preferred Securities [Member]
Investment Securities [Line Items]
Available for sale			$ 8,400,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position | Security			2
Amortized cost			$ 10,500,000
Other-than-temporarily impaired trust preferred securities, amortized cost			2,800,000
Other-than-temporarily impaired trust preferred securities, fair value			$ 2,100,000
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 | Security			1
Gross unrealized losses			$ 1,400,000
Pooled Trust Preferred Securities Sold [Member]
Investment Securities [Line Items]
Available for sale			2,600,000
Second Pooled Trust Preferred Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Gross unrealized losses			610,000
Trust preferred securities [Member]
Investment Securities [Line Items]
Held to maturity, fair value			59,785,000	98,456,000
Available for sale	[1]		$ 8,404,000	$ 20,537,000
Number of available for sale securities (in security) | Security			2	3
Gross unrealized losses			$ 2,054,000	$ 2,953,000
Amortized cost	[1]		10,458,000	23,194,000
Gross unrealized losses	[1]		2,054,000	2,953,000
Obligations of states and political subdivisions [Member]
Investment Securities [Line Items]
Held to maturity, fair value			504,865,000	500,018,000
Available for sale			124,966,000	44,051,000
Gross unrealized losses			441,000	576,000
Amortized cost			125,002,000	44,500,000
Gross unrealized losses			441,000	$ 576,000
Obligations of states and political subdivisions [Member] | Other Than Temporarily Impaired Securities [Member]
Investment Securities [Line Items]
Available for sale			18,500,000
Amortized cost			$ 17,600,000
Investment carrying value percentage			50.00%
Investment carrying value			$ 629,800,000
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Investment Securities [Line Items]
Available for sale			$ 8,300,000

[1] Includes two and three pooled trust preferred securities, principally collateralized by securities issued by banks and insurance companies, at December 31, 2015 and 2014, respectively.