Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended			12 Months Ended
	Sep. 30, 2012 Y Security	Sep. 30, 2011	Sep. 30, 2012 Y Security	Sep. 30, 2011	Jun. 30, 2012	Dec. 31, 2011 Security	Sep. 30, 2012 Maximum	Sep. 30, 2012 Minimum	Sep. 30, 2012 Trust Preferred Securities Security	Dec. 31, 2011 Trust preferred securities	Sep. 30, 2012 Trust preferred securities	Sep. 30, 2012 Trust preferred securities Investment Grade	Sep. 30, 2012 Trust preferred securities Non-Investment Grade	Sep. 30, 2012 Trust preferred securities Not-Rated	Sep. 30, 2012 Other Debt Obligations [Member]	Sep. 30, 2012 Single-issuer bank trust preferred securities	Dec. 31, 2011 Single-issuer bank trust preferred securities	Sep. 30, 2011 Single-issuer bank trust preferred securities	Sep. 30, 2012 Pooled Trust Preferred Securities Security	Sep. 30, 2012 Pooled Trust Preferred Securities Investment Grade	Sep. 30, 2012 Residential mortgage-backed securities	Dec. 31, 2011 Residential mortgage-backed securities	Sep. 30, 2012 Private Label Mortgage-Backed Securities	Sep. 30, 2012 Private Label Mortgage-Backed Securities Investment Grade	Sep. 30, 2012 Single-issuer bank trust preferred securities	Sep. 30, 2012 Perpetual Preferred Securities Security
Investment Securities [Line Items]
Held to maturity, fair value	$ 1,689,413,000		$ 1,689,413,000			$ 2,027,197,000															$ 886,156,000	$ 1,230,993,000
Available for sale	673,911,000		673,911,000			566,520,000															470,727,000	310,137,000
Trading investment securities	22,104,000		22,104,000			21,938,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position	55		55			43			12										3					1		2
Number of security positions in the securities held to maturity portfolio in an unrealized loss position	26		26			28			10			2	3	5
Fair value of investments held to maturity pledged as collateral	974,700,000		974,700,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years	3.3		3.3
Unrealized losses, available-for-sale securities	20,044,000		20,044,000			49,745,000					5,000,000				8,100,000					6,200,000	3,222,000	5,444,000	1,400,000			1,800,000
Number of private label mortgage-backed securities																							3
Unrealized losses, available-for-sale securities	20,044,000		20,044,000			49,745,000															3,222,000	5,444,000	1,800,000
Amortized Cost	677,371,000		677,371,000			598,837,000					16,500,000								19,900,000		463,489,000	304,631,000				10,000,000
Fair value											11,500,000								11,800,000
Additional estimated credit impairment losses																									4,500,000
Fair value of securities available for sale pledged as collateral	356,200,000		356,200,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years	7.1		7.1
Impairment losses on securities	4,697,000		5,247,000	825,000						18,300,000
Additional estimated credit losses	4,500,000
Other-than-temporarily impaired trust preferred securities, amortized cost			41,800,000	47,200,000
Non-credit impairment charges (before taxes)			4,500,000
Fair value of combined securities declined							5,300,000	37,300,000
Fair value of securities					42,600,000
Other-than-temporarily impaired mortgage-backed securities, amortized cost			43,000,000
Other-than-temporarily impaired mortgage-backed securities, fair value	41,500,000		41,500,000
Other-than-temporarily impaired trust preferred securities, fair value		40,800,000		40,800,000
Gross loss on the early redemption	5,300,000
Fair value of trading securities																22,100,000	21,900,000	22,100,000
Interest income on trading securities	$ 442,000	$ 500,000	$ 1,300,000	$ 1,600,000