Equity (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of restricted stock
Compensation expense related to long-term compensation plans, restricted stock and stock options capitalized to assets	$ 3,400,000	$ 2,200,000	$ 1,700,000
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
Weighted average assumptions used for estimation of fair value of each stock option
Dividend yield (as a percent)	2.00%	2.00%	2.15%
Expected life of option (in years)	4.2	5.1	5
Risk-free interest rate (as a percent)	1.00%	2.09%	2.17%
Expected stock price volatility (as a percent)	47.98%	50.07%	53.08%
Weighted average fair value of awards granted during the year	4,647,554	4,333,281	8,276,500
Options Outstanding
Balance at beginning of year (in shares)	1,353,002	1,324,221	937,706
Granted (in shares)	212,400	180,250	443,850
Exercised (in shares)	(243,901)	(109,636)	(22,000)
Lapsed or cancelled (in shares)	(44,301)	(41,833)	(35,335)
Balance at end of period (in shares)	1,277,200	1,353,002	1,324,221
Options exercisable at end of period (in shares)	644,429	631,224	595,851
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 58.85	$ 56.74	$ 61.33
Granted (in dollars per share)	$ 66.42	$ 62.00	$ 46.08
Exercised (in dollars per share)	$ 40.48	$ 31.49	$ 28.17
Lapsed or cancelled (in dollars per share)	$ 65.89	$ 77.33	$ 62.75
Balance at end of period (in dollars per share)	$ 63.37	$ 58.85	$ 56.74
Options exercisable at end of period (in dollars per share)	$ 72.31	$ 69.42	$ 62.17
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)	4.0
Remaining weighted average contractual life of the options exercisable (in years)	4.0
Share-based compensation, additional disclosures
Compensation expense	4,700,000	4,400,000	2,800,000
Total unrecognized compensation cost related to unvested stock awards	8,400,000
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 year	21,768,084	28,269,983	4,979,218
Share-based compensation, additional disclosures
Compensation expense	17,365,401	15,327,206	23,301,744
Total unrecognized compensation cost related to unvested stock awards	14,700,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)	2
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)	2,728,290	2,330,532	1,824,190
Granted (in shares)	185,333	400,925	506,342
Cancelled (in shares)	(1,167)	(3,167)
Balance at end of period (in shares)	2,912,456	2,728,290	2,330,532
Vested during the period (in shares)	66,299	153,644	420,050
Fair value of restricted stock vested during the period	4,300,000	16,600,000	28,000,000
LTIP units
Weighted average assumptions used for estimation of fair value of each stock option
Weighted average fair value of awards granted during the year	$ 8,500,000