Stockholder's Equity (Summary of Common Stock Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares
Shares Issued
Dividend reinvestment plan issuances (in shares) | shares	5,798,979	6,529,417	5,640,008
Option exercises (in shares) | shares	10,736	56,960	252,979
Equity shelf program issuances (in shares) | shares	7,855,956	5,241,349	2,986,574
Preferred stock conversions (in shares) | shares	12,712,452	83	4,300
Redemption of equity membership units (in shares) | shares			91,180
Stock incentive plans, net of forfeitures (in shares) | shares	203,889	115,243	154,337
Totals (in shares) | shares	26,582,012	11,943,052	9,129,378
Average Price
Dividend reinvestment plan issuances (in USD per share)	77.18	65.55	70.13
Option exercises (in USD per share)	51.32	42.66	51.16
Equity shelf program issuances (in USD per share)	78.15	69.95	72.30
Gross Proceeds
Dividend reinvestment plan issuances	$ 447,559	$ 428,009	$ 395,526
Option exercises	551	2,430	12,942
Equity shelf program issuances	613,948	366,640	215,917
Totals	1,062,058	797,079	624,385
Net Proceeds
Dividend reinvestment plan issuances	443,929	423,075	394,639
Option exercises	551	2,430	12,942
Equity shelf program issuances	611,645	364,070	214,406
Totals	$ 1,056,125	$ 789,575	$ 621,987