Stockholder's Equity (Details 3) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares
Class of Stock [Line Items]
Dividend reinvestment plan issuances (in shares) | shares	182,910	2,836,216
Dividend reinvestment plan issuances, average price (USD per share) | $ / shares	55.40	70.55
Dividend reinvestment plan issuances, Gross Proceeds | $	$ 10,133	$ 200,097
Proceeds from dividend reinvestment plan | $	$ 10,133	$ 199,757
Option exercises, shares (in shares) | shares	1,026	202,190
Option exercises, average price (USD per share) | $ / shares	53.61	50.88
Option exercises, Gross Proceeds | $	$ 55	$ 10,288
Option exercises, Net Proceeds | $	$ 55	$ 10,288
Equity shelf plan issuances, shares issued (in shares) | shares		2,986,574
Equity shelf plan issuances, average price (USD per share) | $ / shares		72.30
Equity Shelf Plan Issuances Gross Proceeds | $		$ 215,917
Equity Shelf Plan Issuances Net Proceeds | $		$ 214,406
Shared issued for redemption of units (in shares) | shares		91,180
Preferred stock conversions (in shares) | shares	95	0
Stock incentive plans, net of forfeitures, shares issued (in shares) | shares	114,037	159,709
Issuance of Common Stock (in shares) | shares	298,056	6,275,869
Gross Proceeds From Issuance of Common Stock | $	$ 10,188	$ 426,302
Net proceeds from the issuance of common stock | $	$ 10,188	$ 424,451
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock conversions (in shares) | shares	83