Equity (Details) - USD ($)				3 Months Ended	12 Months Ended			170 Months Ended
	May 26, 2016	May 26, 2016	May 17, 2016	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	May 19, 2016	Dec. 31, 1995
Stock issued during period, Shares, New issue, Main Offering			1,475,000
Shares Issued During Period, Shares, New Issues, Price Per Share	$ 116.67		$ 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								39,373,745
Treasury Stock, Value, Acquired, Cost Method								$ 1,600,000,000
Accumulated Other Comprehensive Income (Loss), Net of Tax					$ (9,828,000)	$ (10,538,000)		$ (9,828,000)
Treasury Stock Acquired, Average Cost Per Share								$ 41.20
Treasury Stock, Shares, Retired					1,250,000			33,475,406
Treasury Stock, Retired, Cost Method, Amount					$ 0	$ 0	$ 0	$ 1,200,000,000
Treasury Stock, Shares					644,991	1,974,097		644,991
Stock Repurchase Program, Remaining Authorized Repurchase Amount					3,834,634			3,834,634
Common Stock, Dividends, Per Share, Cash Paid					$ 0.55
Dividends					$ (95,145,000)	$ (94,235,000)	$ (89,089,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax					600,000	661,000		$ 600,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax					$ (9,769,000)	$ (10,201,000)		$ (9,769,000)
Subsequent Event [Member]
Treasury Stock, Shares, Acquired				189
Treasury Stock, Value, Acquired, Cost Method				$ 22,800
Common Stock, Dividends, Per Share, Declared				$ 0.55