Stockholder's Equity (Details 2) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	5,640,008	4,145,457	4,024,169
Dividend reinvestment plan issuances, Average Price	$ 69.97	$ 70.34	$ 67.72
Dividend reinvestment plan issuances, Gross Proceeds	$ 395,526	$ 291,852	$ 272,531
Dividend reinvestment plan issuances, Net Proceeds	$ 394,639	$ 291,571	$ 272,531
Equity shelf program issuances, Shares Issued	2,986,574	3,134,901	696,070
Equity shelf program issuances, Average Price	$ 71.79	$ 75.27	$ 69.23
Equity shelf program issuances, Gross Proceeds	$ 215,917	$ 238,286	$ 48,186
Equity shelf program issuances, Net Proceeds	$ 214,406	$ 235,959	$ 47,463
Option exercises, Shares	252,979	141,405	249,054
Option exercises, Average Price	$ 51.16	$ 47.13	$ 47.35
Option exercises, Gross Proceeds	$ 12,942	$ 6,664	$ 11,793
Option exercises, Net Proceeds	$ 12,942	$ 6,664	$ 11,793
Senior note conversions			1,330,474
Preferred stock conversions	4,300
Issuance of Common Stock, Shares	9,129,378	7,824,503	25,987,604
Gross Proceeds From Issuance Of Common Stock	$ 624,385	$ 536,802	$ 1,808,535
Net proceeds from the issuance of common stock	$ 621,987	$ 534,194	$ 1,755,722
Stock incentive plans, net of forfeitures, Shares Issued	154,337	402,740	137,837
Redemption of equity membership units	91,180
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
Public issuance, Net Proceeds			$ 1,423,935