Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended			146 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Treasury Stock, Shares, Acquired					39,311,745
Treasury Stock, Value, Acquired, Cost Method					$ 1,600,000
Accumulated Other Comprehensive Income (Loss), Net of Tax		$ (10,538)	$ (11,894)		$ (11,894)
Treasury Stock Acquired, Average Cost Per Share					$ 41.08
Treasury Stock, Shares, Retired		1,250,000			32,225,406
Treasury Stock, Retired, Cost Method, Amount		$ 0	$ 0	$ 0	$ 1,049,400
Treasury Stock, Shares		1,974,097	1,963,018		1,963,018
Stock Repurchase Program, Remaining Authorized Repurchase Amount		2,460,903
Common Stock, Dividends, Per Share, Cash Paid		$ 0.55
Dividends		$ (94,235)	$ (89,089)	$ (58,642)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax		661	718		$ 718
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Transition Assets (Obligations), before Tax		0	65		65
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax		$ (10,201)	$ (11,595)		$ (11,595)
Subsequent Event [Member]
Treasury Stock, Shares, Acquired	270
Treasury Stock, Value, Acquired, Cost Method	$ 0
Common Stock, Dividends, Per Share, Declared	$ 0.55