Stockholders' Equity - Common Stock (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares
Shares Issued
Dividend reinvestment plan issuances (in shares) | shares	4,304,712	182,910
Option exercises (in shares) | shares	10,736	1,026
Equity Shelf Program issuances (in shares) | shares	4,384,045
Stock incentive plans, net of forfeitures (in shares) | shares	167,565	114,037
Totals (in shares) | shares	21,579,510	298,056
Average Price
Dividend reinvestment plan issuances (in USD per share) | $ / shares	75.20	55.40
Option exercises (in USD per share) | $ / shares	51.32	53.61
Equity shelf plan issuances (in USD per share) | $ / shares	74.97
Gross Proceeds
Dividend reinvestment plan issuances	$ 323,724	$ 10,133
Option exercises	551	55
Equity shelf plan issuances	328,665
Totals	652,940	10,188
Net Proceeds
Dividend reinvestment plan issuances	320,243	10,133
Option exercises	551	55
Equity shelf plan issuances	326,362
Totals	$ 647,156	$ 10,188
Convertible Preferred Stock
Shares Issued
Preferred stock conversions (in shares) | shares	12,712,452	83