Performance Share Unit Activity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shares
Granted	100,000	200,000	200,000
Weighted-Average Grant Date Fair Value
Unvested at January 1, 2014	$ 50.33
Granted	$ 74.10
Vested	$ 45.10
Forfeited	$ 57.83
Unvested at December 31, 2014	$ 59.71	$ 50.33
Performance Shares
Shares
Unvested at January 1, 2014	53,205
Granted	91,030	91,030
Vested	0
Forfeited	(8,695)
Unvested at December 31, 2014	135,540	53,205
Weighted-Average Grant Date Fair Value
Unvested at January 1, 2014	$ 59.98
Granted	$ 94.55
Vested	$ 0.00
Forfeited	$ 78.26
Unvested at December 31, 2014	$ 81.87	$ 59.98