Shareholders' Equity (Details) - USD ($)		3 Months Ended			6 Months Ended
	Jan. 03, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Number of shares authorized to be repurchased		7,100,000			7,100,000
Purchase of treasury stock		$ 13,000,000	$ 0	$ 3,700,000		$ 95,100,000
Treasury Stock, Shares, Acquired		300,000		100,000		2,800,000
Treasury Stock Acquired, Average Cost Per Share		$ 43.83		$ 29.70		$ 33.79
Common stock reissued					1,700,000
City Securities [Member]
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Stock Issued During Period Shares Acquisitions	200,000