Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions					1 Months Ended	12 Months Ended
	Feb. 21, 2019	Aug. 31, 2018	Jan. 03, 2017	Jul. 11, 2016	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Number of shares authorized to be repurchased						5,100,000
Purchase of treasury stock						$ 215.4	$ 170.2
Treasury stock acquired, Shares,						3,900,000	3,400,000
Treasury Stock Acquired, Average Cost Per Share						$ 54.94	$ 49.59
Common stock reissued						1,400,000	500,000
Preferred stock, par value						$ 1	$ 1	$ 1.00
Non-Cumulative Perpetual Preferred Stock, Series B [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of preferred stock	$ 150.0
Preferred stock, dividend rate percentage	6.25%
Preferred stock, par value	$ 1.00
Preferred stock liquidation preference per depositary share	25
Preferred stock, liquidation preference per share	$ 25,000
Preferred stock, redemption terms						The Company may redeem the Series B preferred stock at its option, subject to regulatory approval, on or after March 15, 2024, or following a regulatory capital treatment event, as defined.
Non-Cumulative Perpetual Preferred Stock, Series A [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of preferred stock				$ 150.0
Preferred stock, dividend rate percentage				6.25%
Preferred stock, par value				$ 1.00
Preferred stock liquidation preference per depositary share				25
Preferred stock, liquidation preference per share				$ 25,000
Series B Preferred Stock [Member] | Over-Allotment Option [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of preferred stock					$ 10.0
Business Bancshares, Inc. [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Stock issued for acquisition		2,000,000.0
City Securities [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Stock issued for acquisition			200,000