Segment Information (Company's Segment Information, Continuing Operations) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 187,787	$ 192,607
Operations and maintenance expense	69,128	73,541
Depreciation	33,837	32,145
Operating income (loss)	69,896	72,331
Interest expense, net	21,326	19,853
Allowance for funds used during construction	3,193	2,308
Income tax expense (benefit)	2,930	3,007
Net income (loss)	49,072	51,737
Capital expenditures	94,562	72,984
Regulated [Member]
Segment Reporting Information [Line Items]
Operating revenues	186,349	186,006
Operations and maintenance expense	67,510	67,325
Depreciation	33,666	32,206
Operating income (loss)	71,067	72,850
Interest expense, net	19,777	18,801
Allowance for funds used during construction	3,193	2,308
Income tax expense (benefit)	3,856	3,673
Net income (loss)	50,896	52,894
Capital expenditures	94,409	72,389
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,438	6,601
Operations and maintenance expense	1,618	6,216
Depreciation	171	(61)
Operating income (loss)	(1,171)	(519)
Interest expense, net	1,549	1,052
Allowance for funds used during construction
Income tax expense (benefit)	(926)	(666)
Net income (loss)	(1,824)	(1,157)
Capital expenditures	$ 153	$ 595