Stockholder's Equity (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	4,122,941	3,429,928	2,136,140
Dividend reinvestment plan issuances, Average Price	$ 62.35	$ 62.78	$ 56.37
Dividend reinvestment plan issuances, Gross Proceeds	$ 257,055,000	$ 215,346,000	$ 120,411,000
Dividend reinvestment plan issuances, Net Proceeds	257,055,000	215,346,000	120,411,000
Option exercises, Shares	498,549,000	213,724	341,371
Option exercises, Average Price	$ 45.79	$ 42.16	$ 40.86
Option exercises, Gross Proceeds	22,831,000	9,010,000	13,949,000
Option exercises, Net Proceeds	22,831,000	9,010,000	13,949,000
Senior note conversions	258,542	988,007	1,039,721
Equity issued in acquisition of noncontrolling interest		1,108,917
Preferred stock conversions	233,236	116,618
Issuance of Common Stock, Shares	39,226,415	28,857,194	67,917,232
Gross Proceeds From Issuance Of Common Stock	2,420,065,000	1,914,856,000	3,726,960,000
Net proceeds from the issuance of common stock	2,343,868,000	1,854,637,000	3,581,292,000
Stock incentive plans, net of forfeitures, Shares Issued	188,147
February 2012 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues			20,700,000
Public issuance, Average Price			$ 53.5
Public issuance, Gross Proceeds			1,107,450,000
Public issuance, Net Proceeds			1,062,256,000
August 2012 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues			13,800,000
Public issuance, Average Price			$ 58.75
Public issuance, Gross Proceeds			810,750,000
Public issuance, Net Proceeds			778,011,000
September 2012 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues			29,900,000
Public issuance, Average Price			$ 56
Public issuance, Gross Proceeds			1,674,400,000
Public issuance, Net Proceeds			1,606,665,000
May 2013 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues		23,000,000
Public issuance, Average Price		$ 73.5
Public issuance, Gross Proceeds		1,690,500,000
Public issuance, Net Proceeds		1,630,281,000
June 2014 public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues	16,100,000
Public issuance, Average Price	$ 62.35
Public issuance, Gross Proceeds	1,003,835,000
Public issuance, Net Proceeds	968,517,000
September 2014 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues	17,825,000
Public issuance, Average Price	$ 63.75
Public issuance, Gross Proceeds	1,136,344,000
Public issuance, Net Proceeds	$ 1,095,465,000