Summary of Significant Accounting Policies (Detail) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2014 USD ($) $ / shares	Sep. 30, 2015 USD ($) item $ / shares	Sep. 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	$ 234,680		$ 234,680		$ 233,811
Affordable Housing Investments	222,877		222,877		186,423
Other Assets	553,390		553,390		537,847
Shareholders' equity
Retained earnings	2,182,259		2,182,259		2,071,230
Noninterest Expense
Other operating	11,200	$ 10,927	31,713	$ 29,259
Income taxes	32,208	37,452	100,489	103,571
Net income	$ 71,729	$ 70,738	$ 202,827	$ 191,238
Net income per common share, basic (in dollars per share) | $ / shares	$ 1.20	$ 1.18	$ 3.38	$ 3.19
Net income per common share, diluted (in dollars per share) | $ / shares	$ 1.18	$ 1.17	$ 3.32	$ 3.15
Cash Flows From Operating Activities
Net income	$ 71,729	$ 70,738	$ 202,827	$ 191,238
Adjustments to net income:
Deferred income tax benefit			(7,860)	(3,391)
Other, net			27,570	22,063
Net change in:
Other assets and other liabilities, net			$ (18,192)	(44,520)
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		15,215		41,628
Income taxes		34,404		90,521
Net income		$ 69,498		$ 191,919
Net income per common share, basic (in dollars per share) | $ / shares		$ 1.16		$ 3.20
Net income per common share, diluted (in dollars per share) | $ / shares		$ 1.15		$ 3.16
Cash Flows From Operating Activities
Net income		$ 69,498		$ 191,919
Adjustments to net income:
Deferred income tax benefit				(3,758)
Other, net				21,706
Net change in:
Other assets and other liabilities, net				(44,477)
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		10,927		29,259
Income taxes		37,452		103,571
Net income		$ 70,738		$ 191,238
Net income per common share, basic (in dollars per share) | $ / shares		$ 1.18		$ 3.19
Net income per common share, diluted (in dollars per share) | $ / shares		$ 1.17		$ 3.15
Cash Flows From Operating Activities
Net income		$ 70,738		$ 191,238
Adjustments to net income:
Deferred income tax benefit				(3,391)
Other, net				22,063
Net change in:
Other assets and other liabilities, net				$ (44,520)