Equity (Details) - USD ($)				3 Months Ended	12 Months Ended			182 Months Ended
	May 26, 2016	May 26, 2016	May 17, 2016	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	May 18, 2017	Dec. 31, 1995
Stock issued during period, Shares, New issue, Main Offering			1,475,000.0
Shares Issued During Period, Shares, New Issues, Price Per Share			$ 116.67
Stock issued during period, Shares, New Issues, Additional Offering		221,250
Stock Issued During Period, Shares, New Issues	1,696,250
Stock Issued During Period, Shares, New Issues, Net Proceeds	$ 197,200,000
Stock Repurchase Program, Number of Shares Authorized to be Repurchased, Percent									10.00%	10.00%
Treasury Stock, Shares, Acquired								39,532,314
Treasury Stock, Value, Acquired, Cost Method								$ 1,600,000,000
Accumulated Other Comprehensive Income (Loss), Net of Tax					$ (8,353,000)	$ (9,828,000)		$ (8,353,000)
Treasury Stock Acquired, Average Cost Per Share								$ 41.45
Treasury Stock, Shares, Retired								33,475,406
Treasury Stock, Retired, Cost Method, Amount						$ 0	$ 0	$ 1,200,000,000
Treasury Stock, Shares					612,047	644,991		612,047
Stock Repurchase Program, Remaining Authorized Repurchase Amount					3,867,578			3,867,578
Common Stock, Dividends, Per Share, Cash Paid					$ 0.55
Dividends					$ (97,143,000)	$ (95,145,000)	$ (94,235,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax					541,000	600,000		$ 541,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax					$ (8,703,000)	$ (9,769,000)		$ (8,703,000)
Subsequent Event [Member]
Treasury Stock, Shares, Acquired				20,032
Treasury Stock, Value, Acquired, Cost Method				$ 2,241,200
Common Stock, Dividends, Per Share, Declared				$ 0.55