Stockholder's Equity (Summary of Common Stock Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued (in shares) | shares	6,529,417	5,640,008	4,145,457
Dividend reinvestment plan issuances, Average Price (in USD per share)	65.55	70.13	70.40
Dividend reinvestment plan issuances, Gross Proceeds	$ 428,009	$ 395,526	$ 291,852
Dividend reinvestment plan issuances, Net Proceeds	$ 423,075	$ 394,639	$ 291,571
Option exercises, Shares Issued (in shares) | shares	56,960	252,979	141,405
Option exercises, Average Price (in USD per share)	42.66	51.16	47.13
Option exercises, Gross Proceeds	$ 2,430	$ 12,942	$ 6,664
Option exercises, Net Proceeds	$ 2,430	$ 12,942	$ 6,664
Equity shelf program issuances, Shares Issued (in shares) | shares	5,241,349	2,986,574	3,134,901
Equity shelf program issuances, Average Price (in USD per share)	69.95	72.30	76.01
Equity shelf program issuances, Gross Proceeds	$ 366,640	$ 215,917	$ 238,286
Equity shelf program issuances, Net Proceeds	$ 364,070	$ 214,406	$ 235,959
Preferred stock conversions, Shares Issued (in shares) | shares	83	4,300
Redemption of equity membership units, Shares Issued (in shares) | shares		91,180
Stock incentive plans, net of forfeitures, Shares Issued (in shares) | shares	115,243	154,337	402,740
Shares Issued (in shares) | shares	11,943,052	9,129,378	7,824,503
Gross Proceeds	$ 797,079	$ 624,385	$ 536,802
Net Proceeds	$ 789,575	$ 621,987	$ 534,194