Stockholder's Equity (Details 3) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares
Class of Stock [Line Items]
Dividend reinvestment plan issuances (in shares) | shares	1,755,446	4,312,447
Dividend reinvestment plan issuances, average price (USD per share) | $ / shares	64.24	71.14
Dividend reinvestment plan issuances, Gross Proceeds | $	$ 112,770	$ 306,785
Proceeds from dividend reinvestment plan | $	$ 112,294	$ 305,996
Option exercises, shares (in shares) | shares	32,120	209,192
Option exercises, average price (USD per share) | $ / shares	39.94	50.62
Option exercises, Gross Proceeds | $	$ 1,283	$ 10,590
Option exercises, Net Proceeds | $	$ 1,283	$ 10,590
Equity shelf plan issuances, shares issued (in shares) | shares	1,944,511	2,986,574
Equity shelf plan issuances, average price (USD per share) | $ / shares	66.72	72.30
Equity Shelf Plan Issuances Gross Proceeds | $	$ 129,744	$ 215,917
Equity Shelf Plan Issuances Net Proceeds | $	$ 128,834	$ 214,406
Shared issued for redemption of units (in shares) | shares		91,180
Preferred stock conversions (in shares) | shares	95	4,935
Stock incentive plans, net of forfeitures, shares issued (in shares) | shares	112,868	135,773
Issuance of Common Stock (in shares) | shares	3,845,028	7,739,462
Gross Proceeds From Issuance of Common Stock | $	$ 243,797	$ 533,292
Net proceeds from the issuance of common stock | $	$ 242,411	$ 530,992
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock conversions (in shares) | shares	83	4,296