Acquisitions (Narrative) (Details) shares in Millions						12 Months Ended
	Oct. 01, 2018 USD ($)	Aug. 31, 2018 USD ($) shares	Mar. 19, 2018 USD ($) ClientAdvisor State	May 03, 2016 USD ($)	Jan. 04, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 03, 2017 USD ($)	Dec. 04, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill						$ 1,034,679,000	$ 968,834,000
Restricted stock units, charges at the date of modification							55,900,000
Restricted Stock Units (RSUs) [Member]
Business Acquisition [Line Items]
Restricted stock units, charges at the date of modification								$ 58,600,000
Global Wealth Management [Member]
Business Acquisition [Line Items]
Goodwill						340,395,000	276,477,000
Rand & Associates [Member]
Business Acquisition [Line Items]
Period of contingent consideration	5 years
Liability for earn-out payments						$ 4,100,000
Rand & Associates [Member] | Global Wealth Management [Member]
Business Acquisition [Line Items]
Goodwill	$ 8,300,000
Rand & Associates [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Identifiable intangible assets	$ 4,600,000
Business Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Date of acquisition		Aug. 31, 2018
Business acquisition, equity interest issued or issuable, description						Under the terms of the merger agreement, each outstanding share of BBI common stock (except for shares of BBI common stock held by BBI as treasury stock) were converted into the right to receive 0.705 shares of our company’s common stock, with fractional shares settled with cash.
Business combination equity interest exchange ratio		0.705
Shares issued for acquisition | shares		2.0
Acquisition of loans		$ 507,800,000
Acquisition of other assets		85,700,000
Acquisition of deposits		501,100,000
Acquisition of other liabilities		21,900,000
Business Bancshares, Inc. [Member] | Global Wealth Management [Member]
Business Acquisition [Line Items]
Goodwill		38,400,000
Business Bancshares, Inc. [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Identifiable intangible assets		$ 8,600,000
Ziegler [Member]
Business Acquisition [Line Items]
Goodwill			$ 19,000,000
Date of acquisition			Mar. 19, 2018
Number of private client advisors | ClientAdvisor			55
Number of states in which private client advisors operates | State			5
Assets			$ 5,000,000,000
Ziegler [Member] | Customer Relationships and Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Identifiable intangible assets			$ 9,500,000
City Securities Corporation [Member]
Business Acquisition [Line Items]
Goodwill									$ 7,100,000
Identifiable intangible assets									$ 4,100,000
ISM Capital LLP [Member]
Business Acquisition [Line Items]
Goodwill				$ 6,300,000
Identifiable intangible assets				$ 2,300,000
Period of contingent consideration				3 years
Liability for earn-out payments						$ 0	1,900,000
Eaton Partners [Member]
Business Acquisition [Line Items]
Goodwill					$ 72,000,000
Identifiable intangible assets					$ 32,300,000
Period of contingent consideration					4 years
Liability for earn-out payments						40,200,000	31,200,000
Barclays Wealth and Investment Management, Americas [Member]
Business Acquisition [Line Items]
Goodwill										$ 24,800,000
Deferred consideration, recognized as a liability						$ 7,400,000	$ 15,200,000
Barclays Wealth and Investment Management, Americas [Member] | Stifel Bank [Member]
Business Acquisition [Line Items]
Assets										600,000,000
Barclays Wealth and Investment Management, Americas [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Identifiable intangible assets										$ 7,300,000