Stockholder's Equity (Details 1)	3 Months Ended
	Mar. 31, 2018 USD ($) shares	Mar. 31, 2017 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares		338,486
Equity shelf plan issuances, average price | $ / shares		69.75
Equity Shelf Plan Issuances Gross Proceeds		$ 23,776,000
Equity Shelf Plan Issuances Net Proceeds		$ 23,609,000
Preferred stock conversions | shares	83
Dividend reinvestment plan issuances, Shares Issued | shares	129,975	1,284,719
Dividend reinvestment plan issuances, Average Price	55.51	68.33
Dividend reinvestment plan issuances, Net Proceeds	$ 7,214,000	$ 87,784,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 7,214,000	$ 87,985,000
Option exercises, Shares | shares		156,675
Option exercises, Average Price		52.71
Option exercises, Gross Proceeds		$ 8,258,000
Option exercises, Net Proceeds		$ 8,258,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	109,046	181,600
Issuance of Common Stock, Shares | shares	239,104	1,961,480
Gross Proceeds From Issuance of Common Stock	$ 7,214,000	$ 120,019,000
Net proceeds from the issuance of common stock	$ 7,214,000	$ 119,651,000