Stockholder's Equity (Summary of Common Stock Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Shares Issued
Dividend reinvestment plan issuances (in shares) | shares	264,153	5,798,979	6,529,417
Option exercises (in shares) | shares	251	10,736	56,960
Equity shelf program issuances (in shares) | shares	6,799,978	7,855,956	5,241,349
Preferred stock conversions (in shares) | shares		12,712,452	83
Stock incentive plans, net of forfeitures (in shares) | shares	281,552	203,889	115,243
Totals (in shares) | shares	7,345,934	26,582,012	11,943,052
Average Price
Dividend reinvestment plan issuances (in USD per share)	72.33	77.18	65.55
Option exercises (in USD per share)	47.81	51.32	42.66
Equity shelf program issuances (in USD per share)	86.48	78.15	69.95
Gross Proceeds
Dividend reinvestment plan issuances	$ 19,105	$ 447,559	$ 428,009
Option exercises	12	551	2,430
Equity shelf program issuances	588,072	613,948	366,640
Totals	607,189	1,062,058	797,079
Net Proceeds
Dividend reinvestment plan issuances	19,105	443,929	423,075
Option exercises	12	551	2,430
Equity shelf program issuances	576,196	611,645	364,070
Totals	$ 595,313	$ 1,056,125	$ 789,575