Stockholder's Equity (Details 1)	3 Months Ended
	Mar. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares	338,486	443,096
Equity shelf plan issuances, average price | $ / shares	69.75	67.12
Equity Shelf Plan Issuances Gross Proceeds	$ 23,776,000	$ 30,192,000
Equity Shelf Plan Issuances Net Proceeds	$ 23,609,000	$ 29,739,000
Dividend reinvestment plan issuances, Shares Issued | shares	1,284,719	1,058,085
Dividend reinvestment plan issuances, Average Price	68.33	60
Dividend reinvestment plan issuances, Net Proceeds	$ 87,784,000	$ 63,484,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 87,985,000	$ 63,484,000
Option exercises, Shares | shares	156,675	9,864
Option exercises, Average Price	52.71	21.29
Option exercises, Gross Proceeds	$ 8,258,000	$ 210,000
Option exercises, Net Proceeds	$ 8,258,000	$ 210,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	181,600	484,005
Issuance of Common Stock, Shares | shares	1,961,480	1,995,050
Gross Proceeds From Issuance of Common Stock	$ 120,019,000	$ 93,886,000
Net proceeds from the issuance of common stock	$ 119,651,000	$ 93,433,000