Stockholders' Equity - Common Stock (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares
Shares Issued
Dividend reinvestment plan issuances (in shares) | shares	4,148,667	129,975
Option exercises (in shares) | shares	2,505
Equity shelf plan issuances (in shares) | shares	3,060,865
Stock incentive plans, net of forfeitures (in shares) | shares	140,940	109,046
Totals (in shares) | shares	20,065,429	239,104
Average Price
Dividend reinvestment plan issuances (in USD per share) | $ / shares	75.04	55.51
Option exercises (in USD per share) | $ / shares	53.89
Equity shelf plan issuances (in USD per share) | $ / shares	74.22
Gross Proceeds
Dividend reinvestment plan issuances	$ 311,301	$ 7,214
Option exercises	135
Equity shelf plan issuances	227,180
Totals	538,616	7,214
Net Proceeds
Dividend reinvestment plan issuances	307,821	7,214
Option exercises	135
Equity shelf plan issuances	225,587
Totals	$ 533,543	$ 7,214
Convertible Preferred Stock
Shares Issued
Preferred stock conversions (in shares) | shares	12,712,452	83