Stockholder's Equity (Details 1) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	766,488	1,143,397
Dividend reinvestment plan issuances, Average Price	$ 76.38	$ 55.71
Dividend reinvestment plan issuances, Net Proceeds	58,547,000	63,703,000
Dividend reinvestment plan issuances, Gross Proceeds	58,547,000	63,703,000
Option exercises, Shares	149,788	1,155
Option exercises, Average Price	47.17	45.02
Option exercises, Gross Proceeds	7,065,000	52,000
Option exercises, Net Proceeds	7,065,000	52,000
Stock incentive plans, net of forfeitures, Shares Issued	166,815
Issuance of Common Stock, Shares	21,681,732	1,377,789
Preferred stock conversions	0	(233,236)
Senior note conversions	1,048,641	1
Equity issued in acquisition of noncontrolling interest		233,236
Gross Proceeds From Issuance of Common Stock	1,541,637,000	63,755,000
Net proceeds from the issuance of common stock	1,489,547,000	63,755,000
February 2015 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued	19,550,000
Public issuance, Average Price	$ 75.5
Public issuance, Gross Proceeds	1,476,025,000
Public issuance, Net Proceeds	$ 1,423,935,000