Stockholders' Equity - Common Stock (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares
Shares Issued
Dividend reinvestment plan issuances (in shares) | shares	4,438,787	1,755,446
Option exercises (in shares) | shares	10,736	32,120
Equity shelf program issuances (in shares) | shares	4,729,045	1,944,511
Stock incentive plans, net of forfeitures (in shares) | shares	192,237	112,868
Totals (in shares) | shares	22,083,257	3,845,028
Average Price
Dividend reinvestment plan issuances (in USD per share) | $ / shares	75.59	64.24
Option exercises (in USD per share) | $ / shares	51.32	39.94
Equity shelf plan issuances (in USD per share) | $ / shares	75.24	66.72
Gross Proceeds
Dividend reinvestment plan issuances	$ 335,535	$ 112,770
Option exercises	551	1,283
Equity shelf plan issuances	355,803	129,744
Totals	691,889	243,797
Net Proceeds
Dividend reinvestment plan issuances	332,054	112,294
Option exercises	551	1,283
Equity shelf plan issuances	353,500	128,834
Totals	$ 686,105	$ 242,411
Convertible Preferred Stock
Shares Issued
Preferred stock conversions (in shares) | shares	12,712,452	83