Segment Information (Company's Segment Information, Continuing Operations) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 243,626			$ 226,137			$ 663,650	$ 632,344
Operations and maintenance expense	82,022			68,624			247,781	216,085
Depreciation	39,489			37,457			118,113	110,037
Amortization	444			199			(2,140)	478
Operating income (loss)	106,470			104,293			254,858	260,384
Interest expense, net	32,643			25,403			92,239	72,664
Interest income	9,680			44			18,117	111
Allowance for funds used during construction	4,613			3,066			12,280	8,510
Equity earnings in joint venture	135			215			1,918	1,508
Provision for income taxes	(1,553)			3,935			(11,894)	1,437
Net income (loss)	88,489	$ 54,903	$ 16,924	78,216	$ 66,590	$ 50,839	160,316	195,645
Capital expenditures							401,558	343,219
Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues	242,856			225,361			661,082	629,692
Operations and maintenance expense	81,078			68,964			229,498	213,314
Depreciation	39,477			37,449			118,014	110,010
Amortization	304			136			(2,560)	327
Operating income (loss)	106,833			103,674			272,596	262,404
Interest expense, net	24,425			22,720			72,170	66,226
Interest income	17			31			37	73
Allowance for funds used during construction	4,613			3,066			12,280	8,510
Provision for income taxes	(2,561)			4,347			291	3,557
Net income (loss)	88,888			80,126			210,285	202,030
Capital expenditures							401,558	343,219
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	770			776			2,568	2,652
Operations and maintenance expense	944			(340)			18,283	2,771
Depreciation	12			8			99	27
Amortization	140			63			420	151
Operating income (loss)	(363)			619			(17,738)	(2,020)
Interest expense, net	8,218			2,683			20,069	6,438
Interest income	9,663			13			18,080	38
Allowance for funds used during construction
Equity earnings in joint venture	135			215			1,918	1,508
Provision for income taxes	1,008			(412)			(12,185)	(2,120)
Net income (loss)	$ (399)			$ (1,910)			(49,969)	(6,385)
Capital expenditures