Stockholder's Equity (Details 1)	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares	2,986,574	3,119,801
Equity shelf plan issuances, average price | $ / shares	72.3	75.27
Equity Shelf Plan Issuances Gross Proceeds	$ 215,917,000	$ 237,131,000
Equity Shelf Plan Issuances Net Proceeds	$ 214,406,000	$ 234,812,000
Redemption of equity membership units | shares	91,180
Preferred stock conversions | shares	4,296
Dividend reinvestment plan issuances, Shares Issued | shares	4,312,447	3,946,821
Dividend reinvestment plan issuances, Average Price	71.14	70.51
Dividend reinvestment plan issuances, Net Proceeds	$ 305,996,000	$ 278,297,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 306,785,000	$ 278,578,000
Option exercises, Shares | shares	209,192	137,579
Option exercises, Average Price	50.62	50.57
Option exercises, Gross Proceeds	$ 10,590,000	$ 6,958,000
Option exercises, Net Proceeds	$ 10,590,000	$ 6,958,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	135,773	442,899
Issuance of Common Stock, Shares | shares	7,739,462	7,647,100
Gross Proceeds From Issuance of Common Stock	$ 533,292,000	$ 522,667,000
Net proceeds from the issuance of common stock	$ 530,992,000	$ 520,067,000