Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 21, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Number of shares authorized to be repurchased			6,800,000		6,800,000
Purchase of treasury stock			$ 71.1	$ 43.0	$ 125.0	$ 45.9
Treasury stock acquired, Shares,			1,300,000	800,000	2,300,000	800,000
Treasury Stock Acquired, Average Cost Per Share			$ 56.32	$ 56.28	$ 54.95	$ 56.32
Preferred stock, par value			$ 1	$ 1.00	$ 1	$ 1.00	$ 1
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
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]
Shares issued					1,300,000