Summary of Significant Accounting Policies (Detail) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2014 USD ($) $ / shares	Jun. 30, 2015 USD ($) item $ / shares	Jun. 30, 2014 USD ($) $ / shares	Dec. 31, 2014 USD ($)
Summary of Significant Accounting Policies
Number of offices for delivery of banking, investment and trust services | item			75
Number of investment advisory affiliates | item			4
Number of unconsolidated subsidiary | item			1
Assets
Deferred tax asset	$ 237,853		$ 237,853		$ 233,811
Affordable Housing Investments	194,443		194,443		186,423
Other Assets	547,612		547,612		537,847
Shareholders' equity
Retained earnings	2,153,871		2,153,871		2,071,230
Noninterest Expense
Other operating	11,660	$ 9,527	20,513	$ 18,332
Income taxes	37,684	35,109	68,281	66,119
Net income	$ 68,979	$ 65,971	$ 131,098	$ 120,500
Net income per common share, basic (in dollars per share) | $ / shares	$ 1.15	$ 1.11	$ 2.17	$ 2.01
Net income per common share, diluted (in dollars per share) | $ / shares	$ 1.13	$ 1.09	$ 2.14	$ 1.98
Cash Flows From Operating Activities
Net income	$ 68,979	$ 65,971	$ 131,098	$ 120,500
Adjustments to net income:
Deferred income tax benefit			(7,905)	(3,492)
Other, net			16,367	17,383
Net change in:
Other assets and other liabilities, net			$ (38,204)	(22,392)
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		13,567		26,413
Income taxes		29,829		56,117
Net income		$ 67,211		$ 122,421
Net income per common share, basic (in dollars per share) | $ / shares		$ 1.13		$ 2.04
Net income per common share, diluted (in dollars per share) | $ / shares		$ 1.11		$ 2.01
Cash Flows From Operating Activities
Net income		$ 67,211		$ 122,421
Adjustments to net income:
Deferred income tax benefit				(2,934)
Other, net				14,926
Net change in:
Other assets and other liabilities, net				(22,414)
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		9,527		18,332
Income taxes		35,109		66,119
Net income		$ 65,971		$ 120,500
Net income per common share, basic (in dollars per share) | $ / shares		$ 1.11		$ 2.01
Net income per common share, diluted (in dollars per share) | $ / shares		$ 1.09		$ 1.98
Cash Flows From Operating Activities
Net income		$ 65,971		$ 120,500
Adjustments to net income:
Deferred income tax benefit				(3,492)
Other, net				17,383
Net change in:
Other assets and other liabilities, net				$ (22,392)