Incentive Plans (Details) (Performance Shares [Member], USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Performance Shares [Member]
Performance Shares
Number of Shares, Nonvested at January 1, 2011	457,571
Number of Shares, Granted	394,770
Number of Shares, Vested	(219,601)
Number of Shares, Canceled/forfeited	(25,201)
Number of Shares, Nonvested at December 31, 2011	607,539	457,571
Weighted-average Grant Date Fair Value, Nonvested at January 1, 2011	$ 42.99
Weighted-average Grant Date Fair Value, Granted	$ 39.38	$ 40.34	$ 45.72
Weighted-average Grant Date Fair Value, Vested	$ 45.72
Weighted-average Grant Date Fair Value, Canceled/forfeited	$ 41.41
Weighted-average Grant Date Fair Value, Nonvested at December 31, 2011	$ 39.73	$ 42.99