Stock-based compensation plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2011
Restricted Stock [Member]
Summary of restricted stock and restricted stock unit activity
Restricted Stock Units Outstanding, Unvested at January 1 , 2011	965,072
Weighted-Average Grant Price, Unvested at January 1 , 2011	$ 52.81
Restricted Stock Outstanding, Granted	451,248
Weighted-Average Grant Price, Granted	$ 85.03
Restricted Stock Outstanding, Vested	(290,034)
Weighted-Average Grant Price, Vested	$ 61.50
Restricted Stock Outstanding, Cancelled	(34,921)
Weighted-Average Grant Price, Cancelled	$ 64.37
Restricted Stock Outstanding, Unvested at December 31, 2011	1,091,365
Weighted-Average Grant Price, Unvested at December 31 , 2011	$ 63.45
Restricted Stock Units (RSUs) [Member]
Summary of restricted stock and restricted stock unit activity
Restricted Stock Units Outstanding, Unvested at January 1 , 2011	719,976
Weighted-Average Grant Price, Unvested at January 1 , 2011	$ 49.99
Restricted Stock Outstanding, Granted	242,282
Weighted-Average Grant Price, Granted	$ 86.35
Restricted Stock Outstanding, Vested	(148,189)
Weighted-Average Grant Price, Vested	$ 46.17
Restricted Stock Outstanding, Cancelled	(12,606)
Weighted-Average Grant Price, Cancelled	$ 73.73
Restricted Stock Outstanding, Unvested at December 31, 2011	801,463
Weighted-Average Grant Price, Unvested at December 31 , 2011	$ 61.31