Equity (Details) (USD $)	0 Months Ended	12 Months Ended
	Sep. 09, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012
Class of Stock [Line Items]
Common stock, shares authorized		280,000,000	280,000,000
Preferred stock, shares authorized		50,000,000	50,000,000
Common stock shares issued in connection with stock options exercised		500,197
Common stock shares issued through dividend reinvestment plan		2,434	2,002	2,331
Common stock shares issued in connection with stock grants		115,163	123,977	96,592
Number of shares of stock grants withheld		55,523	48,310	121,351
Number of shares forfeited		7,970	7,653	7,558
Common shares issued under Employee Stock Purchase Plan		9,848
Proceeds from issuance of common stock		$ 346,134,000	$ 4,703,000	$ 2,430,190,000
Market closing price (in dollars per share)	$ 155.83
Option Indexed to Issuer's Equity, Shares	4,500,000
Forward rate (in dollars per share)	$ 151.74
CEP III
Class of Stock [Line Items]
Common stock shares issued		2,069,538
Maximum value of shares of common stock that can be sold					750,000,000
Period during which common stock can be sold (in months)		36 months
Percentage of compensation received by sales agent		1.50%
Value of additional shares authorized for issuance		346,304,000
Average sales price of shares issued (in dollars per share)		$ 144.95
Proceeds from issuance of common stock		$ 295,465,000