Summary of Significant Accounting Policies (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 item	Mar. 31, 2014	Dec. 31, 2014
Summary of Significant Accounting Policies
Number of offices for delivery of banking, investment and trust services	75
Number of investment advisory affiliates	4
Number of unconsolidated subsidiary	1
Assets
Deferred tax asset	$ 228,273		$ 233,811
Affordable Housing Investments	190,347		186,423
Other Assets	579,189		537,847
Shareholders' equity
Retained earnings	2,109,157		2,071,230
Noninterest Expense
Other operating	8,853	8,805
Income taxes	30,597	31,010
Net income	62,119	54,529
Net income per common share, basic (in dollars per share)	$ 1.03	$ 0.90
Net income per common share, diluted (in dollars per share)	$ 1.01	$ 0.89
Cash Flows From Operating Activities
Net income	62,119	54,529
Adjustments to net income:
Deferred income tax benefit	(3,211)	(3,237)
Other, net	9,583	4,948
Net change in:
Other assets and other liabilities, net	(92,959)	(88,563)
As Reported
Assets
Deferred tax asset			230,376
Affordable Housing Investments			203,010
Other Assets			537,826
Shareholders' equity
Retained earnings			2,084,361
Noninterest Expense
Other operating		12,846
Income taxes		26,288
Net income		55,210
Net income per common share, basic (in dollars per share)		$ 0.91
Net income per common share, diluted (in dollars per share)		$ 0.90
Cash Flows From Operating Activities
Net income		55,210
Adjustments to net income:
Deferred income tax benefit		(2,679)
Other, net		3,720
Net change in:
Other assets and other liabilities, net		(88,574)
As Adjusted
Assets
Deferred tax asset			233,811
Affordable Housing Investments			186,423
Other Assets			537,847
Shareholders' equity
Retained earnings			2,071,230
Noninterest Expense
Other operating		8,805
Income taxes		31,010
Net income		54,529
Net income per common share, basic (in dollars per share)		$ 0.90
Net income per common share, diluted (in dollars per share)		$ 0.89
Cash Flows From Operating Activities
Net income		54,529
Adjustments to net income:
Deferred income tax benefit		(3,237)
Other, net		4,948
Net change in:
Other assets and other liabilities, net		$ (88,563)