Stockholder's Equity (Details 1)	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares	2,986,574	443,096
Equity shelf plan issuances, average price | $ / shares	72.3	67.12
Equity Shelf Plan Issuances Gross Proceeds	$ 215,917,000	$ 30,192,000
Equity Shelf Plan Issuances Net Proceeds	$ 214,406,000	$ 29,739,000
Redemption of OP units | shares	91,180
Dividend reinvestment plan issuances, Shares Issued | shares	2,836,216	1,971,758
Dividend reinvestment plan issuances, Average Price	70.55	64.65
Dividend reinvestment plan issuances, Net Proceeds	$ 199,757,000	$ 127,470,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 200,097,000	$ 127,470,000
Option exercises, Shares | shares	202,190	37,409
Option exercises, Average Price	50.88	48.73
Option exercises, Gross Proceeds	$ 10,288,000	$ 1,823,000
Option exercises, Net Proceeds	$ 10,288,000	$ 1,823,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	159,709	460,047
Issuance of Common Stock, Shares | shares	6,275,869	2,912,310
Gross Proceeds From Issuance of Common Stock	$ 426,302,000	$ 159,485,000
Net proceeds from the issuance of common stock	$ 424,451,000	$ 159,032,000