Capital and Regulatory Matters - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended			1 Months Ended		1 Months Ended				3 Months Ended	12 Months Ended
	Feb. 28, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Mar. 31, 2013 Trust Preferred Securities [Member]	Feb. 28, 2013 Non-Cumulative Perpetual Preferred Stock and Series A Preferred Stock [Member]	Feb. 15, 2013 Non-Cumulative Perpetual Preferred Stock and Series A Preferred Stock [Member]	Feb. 28, 2013 Non-Cumulative Perpetual Preferred Stock [Member]	Feb. 15, 2013 Non-Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Series A Preferred Stock [Member]	Feb. 15, 2013 Series A Preferred Stock [Member]	Mar. 31, 2013 Floating Rate Subordinated Notes [Member]	Dec. 31, 2012 Floating Rate Subordinated Notes [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock value issued include for regulatory tier one capital calculations		$ 144,700,000
Preferred stock dividend rate percentage		5.375%						5.375%
Corporation's aggregate subordinated notes		123,712,000	123,712,000	123,712,000								100,000,000
Trust preferred securities			120,000,000
Percentage of decreases in permissible portion												20.00%
Permissible portion of subordinated note included in capital												60,000,000	80,000,000
Capital to risk-weighted assets ratio excluding proposed phase-out portion of trust preferred securities					14.00%
Leverage ratio excluding proposed phase-out portion of trust preferred securities					8.28%
Preferred stock shares issued		6,000,000	0	0			6,000,000
Preferred stock liquidation preference value							150,000,000			25
Preferred stock liquidation preference		$ 0.01	$ 0.01	$ 0.01					$ 0.01
Preferred stock liquidation preference											$ 25
Net proceeds from the issuance and sale of the Preferred Stock		144,539,000				144,539,000
Accelerated share repurchase program repurchase amount	144,000,000
Accelerated share repurchase program repurchase share	1,900,000
Accelerated share repurchase program repurchase share percent	80.00%
Stock repurchase program authorized amount		150,000,000
Maximum dividends available without prior regulatory approval		$ 216,500,000