Equity (Details 3) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012 y	Jun. 30, 2011	Jun. 30, 2012 y	Jun. 30, 2011	Dec. 31, 2011 y
Summary of restricted stock
Compensation expense related to long-term compensation plans, restricted stock and stock options capitalized to assets	$ 1,000,000	$ 900,000	$ 2,000,000	$ 1,900,000
Stock options
Stock based compensation
Options expiration period (in years)			P10Y
Options vesting period, minimum (in years)			P1Y
Options vesting period, maximum (in years)			P5Y
Period of commencement of option vesting, from date of grant (in years)	1		1
Weighted average assumptions used for estimation of fair value of each stock option
Dividend yield (as a percent)			2.00%		2.00%
Expected life of option (in years)			5.0		4.2
Risk-free interest rate (as a percent)			0.99%		1.00%
Expected stock price volatility (as a percent)			44.00%		47.98%
Weighted average fair value of awards granted during the period			557,944		4,647,554
Options Outstanding
Balance at beginning of year (in shares)			1,277,200	1,353,002	1,353,002
Granted (in shares)			25,000		212,400
Exercised (in shares)			(137,238)		(243,901)
Lapsed or cancelled (in shares)			(25,919)		(44,301)
Balance at end of period (in shares)	1,139,043		1,139,043		1,277,200
Options exercisable at end of period (in shares)	739,287		739,287		644,429
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)			$ 63.37	$ 58.85	$ 58.85
Granted (in dollars per share)	$ 68.16		$ 68.16		$ 66.42
Exercised (in dollars per share)	$ 42.89		$ 42.89		$ 40.48
Lapsed or cancelled (in dollars per share)			$ 66.42		$ 65.89
Balance at end of period (in dollars per share)	$ 65.89		$ 65.89		$ 63.37
Options exercisable at end of period (in dollars per share)	$ 69.32		$ 69.32		$ 72.31
Exercise price of options granted, low end of the range (in dollars per share)			$ 20.67
Exercise price of options granted, high end of the range (in dollars per share)			$ 137.18
Remaining weighted average contractual life of the options outstanding (in years)	3.66		3.66
Remaining weighted average contractual life of the options exercisable (in years)			3.74
Share-based compensation, additional disclosures
Compensation expense	1,100,000	1,000,000	2,800,000	2,600,000
Total unrecognized compensation cost related to unvested stock awards	5,600,000		5,600,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)			3
Restricted Stock Awards
Weighted average assumptions used for estimation of fair value of each stock option
Weighted average fair value of awards granted during the period			116,369		21,768,084
Share-based compensation, additional disclosures
Compensation expense			3,715,237		17,365,401
Total unrecognized compensation cost related to unvested stock awards	7,300,000		7,300,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)			1.5
Annual award vesting rate, low end of range (as a percent)			15.00%
Annual award vesting rate, high end of range (as a percent)			35.00%
Summary of restricted stock
Balance at beginning of year (in shares)			2,912,456	2,728,290	2,728,290
Granted (in shares)			1,694		185,333
Cancelled (in shares)			(200,284)		(1,167)
Balance at end of period (in shares)	2,713,866		2,713,866		2,912,456
Vested during the period (in shares)			306,124		66,299
Fair value of restricted stock vested during the period			16,200,000
LTIP units
Weighted average assumptions used for estimation of fair value of each stock option
Weighted average fair value of awards granted during the period			$ 8,500,000