Shareholders' Equity (Details) - USD ($)		3 Months Ended		9 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Number of shares authorized to be repurchased		6,000,000		6,000,000
Purchase of treasury stock		$ 16,600,000	$ 0	$ 62,500,000	$ 13,000,000
Treasury Stock, Shares, Acquired		300,000		1,100,000	300,000
Treasury Stock Acquired, Average Cost Per Share		$ 53.93		$ 55.66	$ 43.83
Business Bancshares, Inc. [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Shares issued				4,800,000
Common stock reissued				1,100,000
Stock issued for acquisition	2,000,000