Stockholder's Equity (Details 1)	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares	443,096
Equity shelf plan issuances, average price | $ / shares	67.12
Equity Shelf Plan Issuances Gross Proceeds	$ 30,192,000
Equity Shelf Plan Issuances Net Proceeds	$ 29,739,000
Dividend reinvestment plan issuances, Shares Issued | shares	1,058,085	766,488
Dividend reinvestment plan issuances, Average Price	60	76.38
Dividend reinvestment plan issuances, Net Proceeds	$ 63,484,000	$ 58,547,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 63,484,000	$ 58,547,000
Option exercises, Shares | shares	9,864	149,788
Option exercises, Average Price	21.29	47.17
Option exercises, Gross Proceeds	$ 210,000	$ 7,065,000
Option exercises, Net Proceeds	$ 210,000	$ 7,065,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	484,005	166,815
Senior note conversions | shares		1,048,641
Issuance of Common Stock, Shares | shares	1,995,050	21,681,732
Gross Proceeds From Issuance of Common Stock	$ 93,886,000	$ 1,541,637,000
Net proceeds from the issuance of common stock	$ 93,433,000	$ 1,489,547,000
February 2015 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued | shares		19,550,000
Public issuance, Average Price | $ / shares		75.5
Public issuance, Gross Proceeds		$ 1,476,025,000
Public issuance, Net Proceeds		$ 1,423,935,000