Equity (Details 2) (USD $)	9 Months Ended			1 Months Ended	3 Months Ended	9 Months Ended	23 Months Ended	1 Months Ended	3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 CEP II and CEP III	Nov. 30, 2010 CEP II	Sep. 30, 2012 CEP II	Sep. 30, 2012 CEP II	Sep. 30, 2012 CEP II	Aug. 31, 2012 CEP III	Sep. 30, 2012 CEP III
Equity
Common stock shares issued			2,165,206		315,323	1,435,215	3,925,980		729,991
Common stock shares issued in connection with stock options exercised	391,387
Common stock shares issued through dividend reinvestment plan	1,830	2,548
Common stock shares issued in connection with stock grants	96,592	499,461
Number of shares of stock grants withheld	120,952	129,176
Restricted common stock shares forfeited	4,027
Options of common stock granted to employees (in shares)	115,303
Maximum value of shares of common stock that can be sold (in dollars)				$ 500,000,000				$ 750,000,000
Period during which common stock can be sold				36 months				36 months
Shares sold, average sales price (in dollars per share)					$ 141.35	$ 140.41	$ 127.36		$ 142.09
Net proceeds from sales of shares (in dollars)	$ 326,653,000	$ 1,037,630,000			$ 43,901,000	$ 198,489,000	$ 492,490,000		$ 102,168,000