Stockholder's Equity (Details 1)	6 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 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	1,971,758	1,766,585
Dividend reinvestment plan issuances, Average Price	64.65	72.83
Dividend reinvestment plan issuances, Net Proceeds	$ 127,470,000	$ 128,653,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 127,470,000	$ 128,653,000
Option exercises, Shares | shares	37,409	211,041
Option exercises, Average Price	48.73	46.26
Option exercises, Gross Proceeds	$ 1,823,000	$ 9,762,000
Option exercises, Net Proceeds	$ 1,823,000	$ 9,762,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	460,047	179,037
Senior note conversions | shares		1,076,971
Issuance of Common Stock, Shares | shares	2,912,310	22,783,634
Gross Proceeds From Issuance of Common Stock	$ 159,485,000	$ 1,614,440,000
Net proceeds from the issuance of common stock	$ 159,032,000	$ 1,562,350,000
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